Which term would replace the underlined word to give the sentence a more positive connotation while not changing the overall meaning?

He is -->(childish) <--and often does things without thinking first.
A. Mature
B. Silly
C. Youthful
D. Irresponsible
(WARNING!) It is not B. Silly, I got it wrong

The term which would replace the underlined word to give the sentence a more positive connotation while not changing the overall meaning is youthful.

Option C.

Explanation:

Connotations are the words which suggests a different meaning meaning in addition to its literal meaning. In the sentence, 'He is childish and often does things without thinking first', if we have to replace the word childish with more positive connotation, then it would be youthful.

Youthful means someone who is childlike, or young. Unlike irresponsible or silly, youthful implies a positive connotation and not the negative one. So by replacing childish with youthful the meaning of the sentence will remain same and it will always add to positive connotation.

the eye wink at the hand; yet let that be / which the eye fears, when it is done, to see.