Correct Answer: buoyant

Solution : The correct choice is the fourth option.

Gloomy describes a state of darkness, sadness, or melancholy. On the other hand, buoyant means cheerful, light-hearted, or optimistic. In the context provided, if Sam is looking gloomy, the opposite would be buoyant, reflecting a more positive and cheerful demeanour.

The meanings of the other options are as follows:

• Sullen denotes being irritated and gloomy, often expressed through silence.
• Morose is characterised by prolonged and habitual gloominess.
• Plain means simple or lacking ornamentation.

Therefore, the correct answer is buoyant.