Answer:
The answer to your question here is=
"The food that was purchased in the lunchroom must be eaten in the classroom."
Step-by-step explanation:
An adjective clause is a dependent clause that, like an adjective, modifies a noun or pronoun. An adjective clause begin with words such as that, when, where, who, whom, whose, which, and why.
An essential (or restrictive) adjective clause provides information that is necessary for identifying the word it modifies.
In your question, 'that' modifies the noun 'food'.
Thats all i know.Hope it helps you