Choose one in English ~ I need an explanation, thank you! The house was very quiet,____as

Select A, "past participle + as it is" is a specific use of the as structure. The house was very quiet, isolated as it was on the side of a mountain. In this sentence, "as it is "(use was in the past tense) emphasizes "isolated", it is the subject in the main clause, here is "house". If you don't want to emphasize it, cancel "as it is", and the sentence becomes: "The house was very quiet, isolated on the side of a mountain".

This structure can also be placed at the beginning of the sentence to become:

Isolated as it was on the side of a mountain, the house was very quiet.