This question tests your understanding of the structure "had better" in English grammar.

The phrase "had better" is a modal idiom used to give strong advice or a warning about the immediate future. It is followed by the base form (infinitive without 'to') of a verb.

Structure:

Subject + had better + base verb

Example: You had better finish your homework.

In the negative form, "not" is placed between "better" and the base verb.

Example: You had better not be late.

In the question, "Hadn't you better" is the negative interrogative form, asking for strong advice. Therefore, it must be followed by the base form of the verb "stay".

Step-by-Step Solution:

Step 1: Identify the key phrase in the sentence: "Hadn't you better".

Step 2: Recall that "had better" is always followed by a base verb.

Step 3: Look at the options: "to stay", "stay", "staying", "stayed".

Step 4: The base form of the verb is "stay".

Final Answer: The most appropriate word to fill in the blank is stay.

The complete sentence is: Hadn't you better stay in with that cold?

Related Topics & Theory:

Modal Verbs and Idioms: "Had better" behaves similarly to a modal verb but is not classified as one. It is a fixed expression used only in this specific form. It is used to talk about actions we think people should do or which are desirable in a specific situation. The meaning is similar to "should" or "ought to", but it is often stronger, implying a negative consequence if the advice is not followed.

Formulae:

Positive Advice: Subject + had better + base verb

Negative Advice: Subject + had better not + base verb

Interrogative: Had + subject + better + base verb...?

Negative Interrogative: Hadn't + subject + better + base verb...?