Correct Answer: judicious

Solution : The correct choice is the second option.

Explanation:
Shrewd means having or showing sharp powers of judgement or being astute in practical matters. Judicious also means having good judgement, making wise decisions, and exercising sound reasoning. Hence, judicious is a synonym for shrewd.

The meanings of the other options are as follows:

• Senseless means lacking sense or comprehension; foolish.
• Frivolous means not having any serious purpose or value.
• Unthinking means acting or doing without careful consideration.

Therefore, the correct answer is judicious.