Energy stored in system or field? I am having difficulty in understanding whether fields store momentum and energy or particles store them or both fields and particles store them?
System of potential energy
In the above source it mentions that energy is stored in system but in this source
Energy stored in fields 
they mention energy is stored in field. So which one is correct?
Please also give an explanation why?
I am trying to ask here if any of the choices I mentioned is correct, why is it correct? Why are other choices wrong? But I am not trying to ask if momentum and energy can exist together. So I feel it's not a duplicate of the question mentioned.
 A: They are both correct, because they talk about a different concept of energy. There are dozens of different concepts of energy in physics. In this case, one is related to another in a subordinate matter; energy of EM field is a generalization, a broader concept than electric potential energy.
Electric potential energy of system of particles is a pre-relativistic concept. Let there be some charged particles in some definite region of space, let us call these particles "a system". Potential energy of this system is sum of contributions that depend solely on positions of the particles in space, or mathematically, on coordinates of these particles in some reference frame; it does not depend on their velocities or other variable quantities. It is a property of the system, but it is not necessarily "distributed" across all points of the region. It is just a number assigned to the system, there is no spatial localization of it implied. In non-relativistic theory, the law of conservation of energy states that sum of kinetic energies of all particles and total potential energy is constant:
$$
E_{kinetic} + E_{potential} = const.,
$$
but where in space exactly this energy "is" is immaterial, all that matters is numerical value of this energy and how it depends on positions and velocities of particles.
This concept of potential energy as function of particle positions works (in the sense of allowing formulation of conservation of energy) in many scenarios that are studied in electrostatics.
It however becomes insufficient whenever induced electric fields or other electromotive forces need to be considered. We know that outside the phenomena of electrostatics, the law of conservation of energy needs to be modified to include other contributions to energy, so it will still hold.
In a relativistic theory such as classical electromagnetic theory, whenever the charged particles move, the potential energy assigned in the usual manner, based on particle positions, doesn't give all electromagnetic energy. In other words, in relativistic EM theory one cannot, in general case where particles move, formulate law of conservation of energy using just kinetic energy of particles and potential energy based on particle positions.
However, Maxwell's equations allow us to formulate the law of local conservation of energy, where all energies, including electromagnetic energy, are assigned to a region of space via a distribution, which gives "density" of that energy type at every point of the region. Total energy of this region can be calculated based on values of these densities there. Thus EM energy is thought of as localized to space, with some definite density. Sometimes it is called EM field energy because in macroscopic EM theory, the distribution at any point of space is a function of total electromagnetic field at that point. 
However, it should be noted that the formula for this electromagnetic energy is still not unique - there is an infinite number of formulae that give valid distribution of EM energy density and all are equivalent, at least in special relativistic setting, where gravity can be ignored.
