Final answer:
The adjective clause in the question is 'unless one is sure he can fulfill the promise' as it modifies the noun 'promise' and provides additional information about it.
Step-by-step explanation:
The adjective clause in the sentence provided by the student is 'unless one is sure he can fulfill the promise'. This clause modifies the noun 'promise' by providing more information about the conditions under which the promise is made. An adjective clause typically starts with a relative pronoun (like who, which, that) or a relative adverb (like when, where, why), and functions as an adjective, modifying a noun or pronoun in the main (independent) clause.
An adjective clause is a type of dependent clause, meaning it cannot stand alone as a complete sentence because it does not express a complete thought. The main clause of the sentence 'It is not right to make a promise' is independent because it can stand alone as a complete thought.