Identify the segment in the sentence which contains a grammatical error.
Going by the number of people present, it was appearing that the seminar was a huge success.

Correct option is D

Option (d) contains an error.
The verb phrase "was appearing" is incorrect because "appear" is a stative verb, which generally does not take continuous forms. The correct structure should be: "it appeared".
Grammatical rule used:
Stative verbs like “know,” “believe,” “like,” “seem,” “appear,” etc., express a state rather than an action and are not normally used in continuous tenses.
Correct sentence:
Going by the number of people present, it appeared that the seminar was a huge success.
Example:
Incorrect: He is appearing nervous.
Correct: He appears nervous.