Identify the complete adjective clause.

Dad is the only one with whom you can discuss this plan.

The adjective clause is "with whom you can discuss this plan ". This clause is made up of a preposition "with " and a relative pronoun "whom".

with whom you can discuss this plan.

Explanation:

Adjective clauses are those that modify, describe or give more information about nouns. We can identify them because they usually begin with a relative pronoun (namely, who, whom, whose, that or which) or a relative adverb (when, where or why). In the sentence, the adjective clause is "with whom you can discuss this plan" because this clause provides information about the noun "dad," and also begins with the relative pronoun "whom," which in this case is accompanied by the preposition "with" ("whom" is sometimes used as the object of a preposition).