Explanation:

- Radiant: This means emitting light or heat, bright or glowing with happiness or energy.

- Option 1: Dismal: Represents a state that is gloomy, dreary, or depressing; not related to radiance.

- Option 2: Frowning: Describes a facial expression showing unhappiness; not synonymous with radiance.

- Option 3: Glowing: Refers to emitting a steady light or exuding warmth or enthusiasm. It aptly matches the meaning of "radiant."

- Option 4: Depressing: Indicates a mood or atmosphere of sadness, not aligning with the bright nature of "radiant."

Your selection of "Glowing" is indeed the best synonym for "Radiant."