The word is " carols "; also, though it's possible to sing to carols, you probably just mean "singing Christmas carols". Otherwise... (b) and (c) are ungrammatical.
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English 1b3Anyway, are b and c incorrect because it appears as though 'whom' refers to church?Yes, that's right.
AvangiI'm not sure I understand how "whom" can refer to the stage at church.It can't; that's why it's incorrect.