Identify the segment in the sentence which contains the grammatical error.…

Solution: The erroneous segment is "studied when".

Why this is incorrect: The word "studied" is the past simple form. After the auxiliary "was" we need the present participle (verb+ing) to form the past continuous tense. Using "was studied" here is ungrammatical for the intended meaning.

Rule: Use past continuous (was/were + verb+ing) to describe an action that was in progress in the past and was interrupted by another action (which is in the simple past).

• Action in progress (past continuous): Rakesh was studying

• Interrupting action (simple past): his friends called him

Correct sentence: Rakesh was studying when his friends called him.