He couldn't comprehend the details of the document.

Answer: Option B

Emphasise : give special importance or value to (something) in speaking or writing.
Grasp : comprehend fully.
Condense : make (something) denser or more concentrated.
Implement : put (a decision, plan, agreement, etc.) into effect.
Comprehend : grasp mentally; understand.

Synonym of Comprehend is Grasp