In order to fill in the blank with the most suitable noun or verb, let's understand what nouns and verbs are.
Nouns are the naming words. For example: Roshni is pretty. Here, 'Roshni' is the noun.
Verbs are the action words. For example: Ram fought bravely. Here, 'Ram' is the noun, and 'fought' is the verb.
NOTE: 'Appear' can be used in the form of a noun as well as a verb.
Option A - 'Appear' can't be used here as it doesn't agree with the subject. As the subject is singular, the verb has to be singular as well. Hence, option A is incorrect.
Option B - 'Appeared' can't be used here, as the second half of the sentence explains to us that the sentence is in the present tense. As 'appeared' is in past tense, we can't use it. Hence, option B is incorrect.
Option C - 'Appearing' is the participle form of the verb. As the sentence is in simple present tense, we can't use the participle here. Thus option C is incorrect.
Option D - 'Appears' is a verb in the singular form and it agrees with the singular subject. Also, it matches the tense of the sentence. Hence, option D is the correct answer.
'It appears like an apple, but it's something else.'