All his friends marvelled at his power of ratiocination.

Answer: 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