Fill in the blank with a suitable option: The doctor recommended that she ___________ in bed for a few days.

The correct answer is: B. Stay

The doctor is recommending that the patient stay in bed for a few days. This is a strong recommendation, and the doctor is using the simple present tense to make it clear that this is what they want the patient to do.

The other options are not as appropriate.

• Will stay is in the future tense, which is not what the doctor is saying. They are not saying that the patient will stay in bed in the future, they are saying that they should stay in bed now.
• Would stay is the past tense, which is also not appropriate. The doctor is not saying that the patient would stay in bed if they had the choice, they are saying that they should stay in bed.
• To stay is the infinitive, which is not used to make recommendations. The doctor is not saying that the patient needs to stay in bed, they are saying that they should stay in bed.

Therefore, the correct answer is B. Stay.