Step 1: Understand the Context.
The sentence states that Mohan performed an action "yesterday," which indicates a past event. Therefore, the verb must be in the past tense to correctly describe the action that occurred in the past.
Step 2: Analyze Each Option.
Option (1): buy — Incorrect, as "buy" is the base form of the verb and does not indicate past tense.
Option (2): bought — Correct, as "bought" is the past tense of "buy," which matches the context of the sentence.
Option (3): will buy — Incorrect, as "will buy" indicates future tense, not past tense.
Option (4): buying — Incorrect, as "buying" is the present participle form and does not indicate past tense.
Step 3: Final Answer.
$$
(2) \mathbf{\text{bought}}
$$