The diagram is for three materials with 3 different thermal conductivities, i think the material with higher thermal conductivity is the number 1 because it transfers the heat much better so it gets to the steady temperature sooner than the other two,am i correct? If not or if my reason isn't correct , i appreciate your helps.
This must represent the average temperature, or the temperature at some specific location within the body (since the temperature will be a function of spatial location and time). Just ask yourself what the curve would look like if the thermal conductivity of the material were zero. This should give you your answer.