Step 1: Read the sentence carefully.
“The trousers which were gifted to me by my father were quite expensive.”
Step 2: An adjectival clause is a clause that describes a noun and usually begins with a relative pronoun like who, which, that.
Step 3: In the given sentence, the noun being described is trousers.
Step 4: The clause starting with the relative pronoun which and giving complete information about the noun is
“which were gifted to me by my father”.
Step 5: Hence, the adjectival clause is
Which were gifted to me by my father.