Tell me why???

When heat is absorbed or emitted by the system at constant pressure,we are actually measuring the change in enthalpy i.e dH=q_p

PLEASE EXPLAIN WITH PROPER REASON...

Enthalpy as being the heat transferred during a constant pressure process. Enthalpy is a very important concept in thermochemistry for the following reasons:Most reactions occur under constant pressure conditions (i.e. rxn’s in open containers such as test tubes and beakers, rxn’s that take place in biological systems, etc.).
Many reactions do not involve gaseous reactants or products, as a consequence there is very little work associated with the reaction and DE ~ DH.