‘Casting aspersions’ is an idiom which means to criticize someone’s character. So, the best fit as meaning of this will be option B ‘Passing slanderous remarks’. Slander means to make false statements. Passing slanderous statements means saying false things about someone which might hamper their reputation. Option A ‘cracking silly jokes’ means joking not so seriously. Option C ‘passing critical remarks’ means making judgement specifically negative. Option D ‘passing left-handed compliments’ means saying something pleasant about a person but that could be an insult also. So, the best fit is Option B which means that we should not pass slanderous remarks on character of our colleagues.