This question tests your understanding of modal verbs and their correct usage in expressing prohibition or strong advice in past situations.

Step 1: Analyze the context
Sentence A describes a past event: "The attendant was smoking near the petrol pump." This action is dangerous and prohibited.

Step 2: Determine the required meaning for Sentence B
Sentence B needs to complete the idea: "He ______________ smoking there." This should express that the action was forbidden or highly inadvisable. Since the smoking already happened, we need a modal verb construction that refers to the past.

Step 3: Evaluate the options

• Shouldn't have been: This is the correct "modal + perfect infinitive" structure ($\mathrm{should}+\mathrm{not}+\mathrm{have}+\mathrm{past}\mathrm{participle}$) used to criticize or express disapproval of a past action. It perfectly fits the context.
• Wouldn't be: This expresses a future-in-the-past or a refusal, not a prohibition of a past action.
• Shouldn't be: This is used for giving advice or stating prohibition about a present or general action, not a specific past event.
• Ought not to be: Similar to "shouldn't be," this is used for present or general advice/prohibition, not for a past action.

Final Answer: The most suitable option is Shouldn't have been.

Sentence B becomes: "He shouldn't have been smoking there." This correctly states that his action of smoking in the past was wrong and prohibited.

Related Topic: Modal Verbs

Modal verbs (e.g., should, would, ought to, must, can) are auxiliary verbs used to express ability, possibility, permission, obligation, or advice. They are often followed by a base verb. To refer to the past, we frequently use the structure Modal + Have + Past Participle (e.g., should have gone, would have seen, must have been).

Key Formula/Structure

Expressing disapproval for a past action:
$\mathrm{Subject}+\mathrm{should}/\mathrm{ought}\mathrm{to}+\mathrm{not}+\mathrm{have}+\mathrm{past}\mathrm{participle}$

Example: You shouldn't have eaten so much cake. (Criticizing a past action)