Calcium is essential for strong bones and teeth. what kind of phrase is the bolded words?

Assuming the bolded words are: "for strong bones and teeth" and that you had to choose between:

noun phrase adverb phrase verb phrase adjective phrase

It is an adjective phrase (4).

An adjective phrase is a group of words complementing, specifying, or modifying a 'head' adjective in the sentence. It is called an adjective (or adjectival) phrase because the whole group of words functions as an adjective, which means it provides information about a noun.

Here, the adjective phrase "for strong bones and teeth" is specifying the head adjective "essential." It is functioning as an adjective for the noun "calcium" by giving us information about it.