CPE reading question

Could someone help with this reading comprehension question from Cambridge Certificate of Proficiency in English 3?

Here's the passage:

Cheney and Seyfarth describe how one day the dominant male in the group of vervet monkeys they were studying noticed a strange male hanging around in a neighbouring grove of trees. The stranger's intentions were quite obvious: he was sizing up the group in order to join it. If he succeeded, it was more than likely that the incument male would be ousted from his position of privilege. With the vervet equivalent of a stroke of a genius, the male hit on the ideal ploy to keep the stranger away from his group.As soon as the strange male descended from the grove trees to try to cross the open ground that sparated his grove from the one in which the group was feeding, he gave an alarm call that vervets use to signal the sighting of a leopard nearby. The stranger shot back into the safety of his trees. As the day wore on, this was repeated every time the strange made a move in the group's direction. All was going swimmingly until the male made a crucual mistake: after successfully using the ploy several times, he gave the leopard alarm call while himself nochalantly walking across the open ground.

20. What aspect of the situation is emphasised by the use of the word 'swimmingly'?

A. the tension
B. the danger
C. the humour
D. the charm

I couldn't decide between C and D. The key says it's C, but why not D? Surely the situation is charming?

Does anyone know what kind of logic lies behind this question? Why is it C?

Thank you in advance.