the whole family slept soundly.

Đáp án A.

Cấu trúc đảo ngữ với mệnh đề though:

N/ V/ Adj/ Adv + though + S + V/ trợ động từ

Child though she is, she knows a great deal.

Try though she does, she will never pass it.

Young though he is, he has won the Nobel Prize.

Hard though he works, he makes little progress.

Vậy đáp án chính xác là A.