We should abstain from casting aspersions on the character of our colleagues.

Answer: Option D

Cracking : excellent.
Slanderous : (of a spoken statement) false and malicious.
Critical : expressing adverse or disapproving comments or judgements.
Throw : propel (something) with force through the air by a movement of the arm and hand.
Casting : throw (something) forcefully in a specified direction.

Synonym of Casting is Throw