The underlined word is 'buoyant'.
In a physical sense, 'buoyant' means able to float.
When used to describe a person's attitude or personality, it metaphorically means light, resilient, and not easily weighed down by negative events.
This translates to being cheerful, optimistic, and lively.
From the given options, 'cheerful' is the closest synonym for a buoyant attitude.
'Hasty' means hurried, 'smart' means intelligent, and 'cruel' means causing pain. None of these fit the context.