7672. All his friends marvelled at his power of ratiocination.

✅ The correct answer is option C.
Ruminating : think deeply about something.

Speculation : the forming of a theory or conjecture without firm evidence.

Imagination : the faculty or action of forming new ideas, or images or concepts of external objects not present to the senses.

Reasoning : the action of thinking about something in a logical, sensible way.

Ratiocination : the process of exact thinking : reasoning.

Synonym of Ratiocination is Reasoning