C. He as a subject because it's in the beginning of a sentence, and a sentence always have to start with a subject
EXPLAINS, IS
The tour guide EXPLAINS that either candles or a lantern IS used to see into the tomb.
Tour guide is singular. Therefore, verb used must be in singular form.
Lantern is singular and it is the nearest subject of the verb. Thus, the verb must also be in singular form.
The answers from the choices above are the 2nd option and the 3rd option!