The Parthenon was a church from 1204 until 1456, when Athens was taken by General Mohammed the Conqueror, the Turkish sultan, who established a mosque in the building and used the Acropolis as a fortress.
Analysis:
The Parthenon (subject) was (verb) a church from 1204 until 1456
- , when (when is used to talk about time - here it is talking about 1456 or period when P ceased to be a church)
(when also starts a sub-ordinate clause)
- Athens (Subject) was (Verb) taken by General Mohammed the Conqueror
- , the Turkish sultan, (non- essential modifier)
- who (modified the conqueror) established (past tense - verb) a mosque in the building
- and used (parallelism - established & used) the Acropolis as a fortress.
No issues. Hence, Option A