Hi hemant_rajput,

This SC begins with an opening (descriptive) phrase followed by a comma, so the first grammar rule that you'll need to follow is based on Modification. We'll need a phrase that correctly describes the subject ("the architect"). Eliminate A and C

With what remains, you have to pay attention to how the verb is presented.

B uses "having known", which sounds awkward and implies that the architect only knew about seniors in the past.
E refers to general knowledge about seniors, but not specifically about what the architect knows.
D offers a descriptive phrase that leads directly into what the architect knows.

GMAT assassins aren't born, they're made,
Rich