Answer:
The answer is: with whom you can discuss this plan.
Step-by-step explanation:
An adjectival or adjective clause is a group of related words that:
1. Contains a verb and a subject.
2. doesn't express a complete thought.
3. describe or modify a noun.
Adjectival clause can be identified most often because they begin with any of the following relative pronouns: who, whose, whom, that or which and sometimes a relative adverb like why, when and where.
In the sentence above, the complete adjectival clause is “with whom you can discuss this plan”. This is because on its own, it does not express a complete thought and it has a subject and a verb.
It's also important to note that though an adjectival clause is a group of words, it still function as an adjective. Meaning it still serves to describe or modify a noun and in this case, the adjectival clause is describing the noun “dad.”