Here's the reasoning behind the correct matches:

(A) Stupas are mounds containing relics of Buddha. Stupas were constructed to house relics of Buddha and are considered sacred Buddhist monuments.

(B) Chaityas are Buddhist shrines. Chaityas are places of worship and meditation in Buddhism, so they are associated with Buddhist shrines.

(C) Harmika is associated with the abode of gods . In Buddhist architecture, harmika is  often used to demarcate and protect sacred areas, and these areas are associated with the abode of gods or divine spaces.

(D) Railings are related to separating the sacred space from the secular world. In Buddhist architecture, the railings a small square railing at the top of a stupa, and it symbolizes the transition from the worldly to the sacred, separating the sacred space from the secular world

So, the matches(A)-(II), (B)-(I), (C)-(IV), (D)-(III)correctly represent the symbolism of these elements in Buddhist monuments.

Therefore, the option [3] is correct.