In the following question, some part of the sentence may have errors. Find out which part of the sentence has an error and select the appropriate option. If a sentence is free from error, select 'No Error'.
Since they were all dressed up, (A)/she assumed they(B)/were going to church together. (C)/No error(D)

The sentence is grammatically correct and conveys the intended meaning without any error.
"Since they were all dressed up" - Clause indicating reason (Correct)

"she assumed they" - Main clause subject + verb + implied relative pronoun 'that' (Correct)

"were going to church together" - Past continuous tense matching the time frame of the first clause (Correct)

Hence, the answer is "No error".