Answer:
An adverb clause
Step-by-step explanation:
An adverb clause is a group of words that have a subject and a verb, and whose function is to act as an adverb, that is to say, to modify adjectives, verbs or other adverbs. This type of clauses often answers questions such as "Why?", "How?", "When?", "Where?" and "In what circumstances?".
In the sentence, "where it would get the most sun" is an adverb clause because it modifies the verb "planted" by giving us information or further detail of WHERE something ("to plant") was done.
Question: Where did you plant the garden?
Answer: Where it would get the most sun.