Answer:
a cucumber
Step-by-step explanation:
The expression as cool as a cucumber means very calm and relaxed, particulary in surprising or unexpected situations. Thus, the man is said to be so peaceful and carefree in the court, where people are expected to feel nervous, that he seemed to be having a good time.
The rest of the options are incorrect because they do not collocate with as cool as to express composure and serenity.