Answer A: the moon is the most brilliant object in the sky.
Sentence A uses correctly the most before the adjective brilliant to make this degree of comparison, the superlative. Brilliant has three sylllables, but if it was a short adjective, the superlative would need the +est instead of most. Examples: tall - the tallest; dark - the darkest etc. In B, there is no comparison. In C the correct use for more is to compare two things/persons. Mary is more beautiful than her sister.
