Correct Answer: Voluntary

Solution : The correct choice is the third option .

Compulsory means something that is required, mandatory, or enforced by rules or laws. It must be done, and there's no choice in the matter. Whereas voluntary refers to something that is done willingly, by choice, or without being forced.

The meaning of other options are as follows:

• Obligatory: Similar to compulsory , something that is obligatory is required, mandatory, or binding, and it must be done.

• Mandatory: It means something that is required or ordered by authority.

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• Subsidiary: This means secondary or subordinate, often in the context of a company or organisation.

Therefore, the correct answer is voluntary.