Not all deserts are sweltering. Option B is the sentence in which the underlined word or group of words is a predicate adjective.
A predicate adjective refers to an adjective possitioned in the predicate, that is the part of the sentence containing the verb, that modifies the subject. In this case, sweltering is the predicate adjective that modifies the noun desert. A predicate adjective is one of many different types of adjectives. Basically, predicate adjectives modify the subject of the sentence. and are always connected to the subject by a linking verb.