An interrogative adjective is a word which both modifies a noun and indicates a question. The words "which" and "what" are used as interrogative adjectives. Whether these words serve as adjectives or merely pronouns, however, depends on whether they modify a noun or stand on their own.
Just as interrogative sentences are questions, interrogative adjectives indicate a question. When a sentence begins with an interrogative adjective, the reader knows a question will be asked. For example, in both "Which car is hers?" and "What river is that?" the adjectives indicate a question is about to be asked.
More importantly, like any adjective, interrogative adjectives modify a noun. Unlike most adjectives, however, the interrogative variety are more limited in scope than their general counterparts. This limitation is partly due to the few interrogative adjectives that exist. With only two options, the resulting scope is rather narrow.
Additionally, although adjectives may add detail and clarification, interrogative adjectives just clarify. For example, in the sentences "Which car is yours?" and "The blue car is mine," the phrases "which car" and "the blue car" both clarify there is a specific car that is in question. Only the phrase "the blue car," however, describes that specific car in any way.
In addition to modifying nouns, an interrogative adjective can modify a noun phrase. A noun phrase is group of words that acts like one noun. For example, in the sentence "Which road should I take?," "road should I" is the noun phrase, "which" acts as the interrogative adjective, and "take" is the verb.
"Which" and "what" are not exclusively interrogative adjectives, however. Depending on their placement in a given sentence, they can be pronouns. Simply rearranging the word order in a sentence may change the adjective to a pronoun. For example, in the sentence "Which answer is correct?," the word "which" is functioning as an interrogative adjective because it is modifying the word "answer." In the sentence, "Which is the correct answer?," however, "which" is no longer modifying "answer," "correct" is now the modifier, so "which" becomes a pronoun.
To determine whether "which" or "what" are pronouns or interrogative adjectives, a reader must first look at the word or phrase that comes directly after the word in question. If the word following "which," for example, is a noun, "which" is most likely an interrogative adjective. On the other hand, if the word following "which" is a verb, "which" is probably serving as a pronoun.