The above exchange shows how to ask what the subject of a verb sentence is (i.e., who/what does or undergoes the action) and how to answer that question. When question words such as だれ and なに are used as the subject of a sentence, they are marked by the particle が. When answering these questions, the subject is also marked by が.
A: だれが きましたか。
B: マナさんが きました。
Mana came./It is Mana who came.
A: なにが はじまりましたか。
B: パーティーが はじまりました。
The party started./It is the party that started.