Step 1: Understanding the Concept:
This question tests the preposition that follows the adjective 'good' to indicate skill or ability in a particular subject or activity.
Step 2: Detailed Explanation:
The fixed expression is "good at something." It is used to say that someone has a high level of skill in a certain area.
- For example: "He is good at mathematics." / "She is good at playing the piano."
While 'good in' is sometimes used in informal speech, 'good at' is the grammatically correct and standard form for indicating ability.
Step 3: Applying to the Sentence:
The sentence describes Sheela's skill in the subject of English. Therefore, the correct preposition is 'at'.
Step 4: Final Answer:
The correct preposition is at.