he was heard_____in the next room.

to sing
heard的用法是
主动:heard sb doing sth
被动:be heard to do
这里是被动,意思为他被听到在隔壁房间唱歌

he was heard_A____in the next room.
--to sing
--sing
--sings
--sang

hear sb to do sth. 听到某人做某事
这里是被动语态
有人听到他在隔壁唱歌

选 A
hear sb do sth ， hear是感官动词，直接带宾语的时候，是带省略了to的不定式。而hear放在被动语态的时候，to必须还原。

to sing