Final answer:
The simile comparing Vilho to 'an exasperated parent' indicates that he cooperates with some reluctance, capturing the essence of option C.
Step-by-step explanation:
The intended effect of the simile in the sentence 'Vilho went along with it, but he was like an exasperated parent humoring a child in a game of make-believe' is most closely aligned with option C. Vilho cooperates but with some reluctance. This is because the comparison to 'an exasperated parent humoring a child' suggests that Vilho is agreeing to participate in something he finds somewhat tiresome or frustrating, much like a parent might indulge a child's game even if they find it uninteresting or it's not convenient at the time. Therefore, the simile conveys a sense of patience and reluctant accommodation, rather than suggesting that Vilho is older, combative, cruel, or childishly imaginative.