In each question four sentences are given in which only one is correct. Choose the statement that is devoid of all grammatical errors. If all are correct, choose, “All are correct” as your answer choice.

Correct option is C

The correct statement is (c)
(a): ‘unless’ must be changed to ‘until’. Unless refers to a condition not being met while until refers to a time threshold not being crossed. This sentence suggests that the boxer was undisputed till a new youngster came and grabbed that title.
(b) "telling them that it is unprofessional to talk on the phone and eating during class" should be changed to "telling them that it is unprofessional to talk on the phone and eat during class." The verb "eat" should be in its base form after "and" to maintain parallelism. Parallelism is a similarity of grammatical form for similar elements of meaning within a sentence or among sentences. If two or more ideas are parallel, they should be expressed in parallel grammatical form.
(d): ‘no’ must be changed to ‘not’.
Not negates the meaning of the modified verb when used as an adverb.
Ex- I do not like playing the cello.
No means "not any" when used as a determiner.
Ex-A good way to remember the difference is that not works with verbs and no works with nouns.