The following sentence has been divided into four segments. Identify the segment that has a grammatical error.
I am working with him since 2002, / but even today / I am unable / to understand his motives.

The correct option containing the grammatical error is: "I am working with him since 2002,"

Step-by-step grammatical analysis:

1. Identify the action and its duration: The sentence expresses an action ("working with him") that began in the past (in the year "2002") and is still continuing into the present (as indicated by the clause "but even today I am unable...").
2. The rule of "since" and "for": In English grammar, when we use the preposition "since" to denote the starting point of an action that continues up to the present, we must use a perfect tense (typically the Present Perfect Continuous or the Present Perfect tense).
3. Identify the error: The segment uses the Present Continuous tense ("I am working"). The Present Continuous tense is used for temporary actions happening right now, but it cannot be paired with a specific duration or starting point like "since 2002".
4. How to correct the error: To make the sentence grammatically correct, the Present Continuous tense "I am working" must be changed to the Present Perfect Continuous tense: "I have been working".
Therefore, the corrected sentence should read: "I have been working with him since 2002, but even today I am unable to understand his motives."