Step 1: Use the collocation comparison method: compare "good in" against other adjective-plus-preposition pairs describing skill, such as "skilled at", "talented at", and "strong at" a subject, all of which pair with "at", never "in".
Step 2: By this pattern, "good", when describing ability in an academic subject, follows the same rule and should be followed by "at": "good at mathematics", not "good in mathematics".
Step 3: Check the remaining options against the rest of the sentence. "Scores well in exams" is correct as written; "scores good" in option (a) would wrongly use the adjective "good" in place of the adverb "well", so that proposed change is itself an error, not a fix. Option (c) suggests removing "always", but the word is grammatically fine and removing it is not required to fix any error. Option (d), "No error", is wrong because the preposition mismatch is a genuine, well-known error.
Step 4: The only real fault in the sentence is the preposition after "good", confirming option (b).
Correct option: (2) 'good in' should be 'good at'.