Explanation:

- Option 1: why his low score

- This segment contains the error. The construction "the reason why" is redundant because "reason" already implies "why". Without "why", the sentence would be: "the reason for his low score."

- Option 2: He wanted to know

- Grammatically correct. The subject and verb agree, forming a correct phrase.

- Option 3: the reason

- This part is correctly structured. It matches with the question related to a cause or explanation.

- Option 4: in Mathematics

- This is correct and correctly specifies the subject of interest—Mathematics.