Question

the suns corona is much hotter than the photosphere, but it is not as bright because the coronas magnetic field prevents the corona from radiating light. the coronal holes allow more light from the photosphere to pass through. the density of the corona is much lower, so the corona does not emit as much light. the granulation on the photosphere allows it to generate more energy.

Explanation:

The brightness of a region depends on its density and the amount of light - emitting material. The Sun's corona has a much lower density compared to the photosphere. Even though it is hotter, with fewer particles per unit volume, it emits less light overall. The magnetic field does not prevent the corona from radiating light, coronal holes are related to solar wind and not the relative brightness, and granulation on the photosphere is not the main reason for the corona's lower brightness.

Answer:

the density of the corona is much lower, so the corona does not emit as much light.