Correct answer:
B. baja.
Here we need to choose an adjective that is the opposite to alta that means tall. Recall that an adjective is a word that gives us more information about a noun or pronoun. So the opposite adjective to tall is small and this translates into bajo/baja in Spanish. Finally, we can write and translate this sentence as follows:
Una persona alta no es baja (A tall person is not small)
