The sentence "Mon frere est _ que moi" requires a comparative construction. In French, comparatives are formed with plus + adjective + que (more ... than), moins + adjective + que (less ... than), or aussi + adjective + que (as ... as).
The correct answer is plus petit, giving the sentence: "Mon frere est plus petit que moi" - "My brother is shorter/smaller than me."
The other options do not form valid comparatives:
plus bon is grammatically incorrect. The comparative of bon (good) is the irregular form meilleur, not plus bon.
petit alone cannot be used in a comparison with que. The word plus (or moins/aussi) is required before the adjective.
le petit is a superlative fragment (le plus petit = the smallest), but it is incomplete and does not fit the ... que moi comparative structure.
Remember: French comparatives always need plus, moins, or aussi before a regular adjective. The only common irregular comparatives are meilleur (better, from bon) and pire (worse, from mauvais).
