This question tests your understanding of modal verbs and their usage in English grammar. Modal verbs express necessity, possibility, permission, or ability. Let's analyze the sentence and the options.

The context is clear: participating in the play requires a costume. However, the second part of the sentence provides a solution for those who cannot afford to buy one ("we will make one"). This means that buying a costume is not an absolute requirement; it is an option, but there is an alternative.

Now, let's evaluate the modal verb options based on their meanings:

Option 1: don't have to
This phrase expresses a lack of obligation. It means something is not necessary. This fits perfectly because the sentence explains that buying is not obligatory since making one is an available alternative.

Option 2: shouldn't
This is used to give advice against doing something (e.g., "You shouldn't eat too much sugar"). It implies that buying the costume is a bad idea, which contradicts the neutral tone of the sentence. The sentence is not advising against buying; it is simply stating it's not required.

Option 3: can't
This expresses inability (e.g., "I can't swim"). If we use this, the sentence would mean "you are unable to buy one," which changes the meaning. The original sentence is about permission/obligation, not ability. The phrase "if you can't afford it" already handles the concept of inability separately.

Option 4: mustn't
This is a strong modal verb used to express prohibition (e.g., "You mustn't tell anyone"). It means you are forbidden from buying a costume. This is too strong and does not fit the helpful context of the sentence, which is offering a solution, not a rule against buying.

Therefore, the word that correctly completes the sentence is don't have to, as it accurately conveys the lack of necessity.

Final Answer:

The best word to complete the sentence is don't have to.
The completed sentence reads: "But, you don't have to buy one. If you can't afford it, we will make one."

Related Topic: Modal Verbs

Modal verbs are auxiliary verbs that modify the main verb to express concepts like necessity, possibility, permission, or ability. Common modal verbs include:

• Must / Have to: Express strong obligation or necessity.
• Mustn't: Expresses prohibition.
• Don't have to / Needn't: Express a lack of obligation (something is not necessary).
• Should / Shouldn't: Used to give advice or make a recommendation.
• Can / Can't: Express ability or permission (in informal contexts).

The key to choosing the right modal verb is to understand the specific nuance (obligation, advice, ability, permission) required by the context of the sentence.