The correct answer is D. No error.
The sentence “As our room was upstairs, so we didn’t hear him” is grammatically correct. The conjunction “so” is used to join two independent clauses that express a cause-and-effect relationship. In this case, the first clause (“As our room was upstairs”) states the cause, and the second clause (“we didn’t hear him”) states the effect.
The sentence could also be written as “Because our room was upstairs, we didn’t hear him.” However, the use of “so” is more concise and direct.
The other options are incorrect because they contain errors in grammar or punctuation.
Option A is incorrect because the comma after “was” is unnecessary.
Option B is incorrect because the word “so” is missing.
Option C is incorrect because the word “didn’t” is missing.