Le lion est fort mais_ est encore plus fort.

The sentence Le lion est fort mais ___ est encore plus fort means "The lion is strong but ___ is even stronger." The blank requires a pronom tonique (stressed/disjunctive pronoun) that functions as the subject of the second clause for emphasis and contrast.

The correct answer is lui. In French, lui is the masculine singular stressed pronoun used for emphasis, contrast, or after prepositions. Here, it creates a contrastive structure: the lion is strong, but he (another entity) is even stronger.

The other pronouns are incorrect for grammatical reasons:

• elle is the feminine stressed pronoun. Since the adjective fort (not forte) is masculine, the pronoun must also be masculine
• nous would require the plural verb form sommes, not the third-person singular est
• moi would require the first-person verb form suis (as in moi, je suis), not est

Only lui is a third-person singular masculine pronoun that agrees with the verb est and the masculine adjective fort.