Identify the segment in the sentence, which contains the grammatical error.
Economics is one of the subject which I have found very difficult since school.

Correct option is D

The segment "one of the subject" contains the grammatical error.
· The correct form should be "one of the subjects," as the prepositional phrase "of the" indicates that the noun that follows should be in its plural form because it refers to one item out of a set or group.
· Grammatical rule: When using the construction "one of the + plural noun," the noun must be in the plural form to denote multiple items where one is being selected.
· Example: Mathematics is one of the subjects I have found very difficult since school, using the plural form to indicate multiple subjects.