Why light water is used as moderator as we know that light water absorbs most of the neutron leading to stop the nuclear fission chain reaction? why most of the nuclear reactor used the light water as moderator although we know that it absorbs most of the neutron which can lead to stopping of nuclear fission chain reaction to stop and if we are using it also then why it is preferred over d2o reactors?
 A: The main reason is on Wikipédia :
" Due to its potential for use in nuclear weapons programs, the possession or import/export of large industrial quantities of heavy water are subject to government control in several countries. Suppliers of heavy water and heavy water production technology typically apply IAEA (International Atomic Energy Agency) administered safeguards and material accounting to heavy water. (In Australia, the Nuclear Non-Proliferation (Safeguards) Act 1987.) In the U.S. and Canada, non-industrial quantities of heavy water (i.e., in the gram to kg range) are routinely available without special license through chemical supply dealers and commercial companies such as the world's former major producer Ontario Hydro."  And we can add  :  the difference of  prices .
A: Light water is a better moderator and allows for smaller reactors for a given power.
Heavy water is not very abundant, and for use in nuclear reactors it must be hyper-pure or you encounter the neutron absorption issues you mention. Heavy water is expensive to produce, and the volume you need is larger (for the larger reactor volume needed-- see the next paragraph.)
As you noted, light water has a higher neutron absorption cross section than D2O so that LWRs require enriched uranium. D2O reactors can use natural uranium but must be much larger to allow for more moderation collisions to thermalize the neutrons.
