Correct order of decreasing thermal stability is as:
The stability of hydrides decreases from ammonia to bismuthine. This is because the central atom E increases in size down the group. With the increase in the size of the central atom, the E – H bond becomes weaker. where E is nitrogen, phosphorus, arsenic, antimony, bismuth.
With the increase in size, the overlapping between the orbitals of E and hydrogen will not be effective. This accounts for the decrease in the stability of hydrides as we move down the group.
The correct order is :NH3>PH3>AsH3>SbH3
Hence, option A is correct.