Select the most appropriate option to fill in the blank.

If nature ______ to degrade, the world’s economic output will suffer sooner or later.

Continues

The most appropriate option to fill in the blank is continues.

The sentence is in the present tense, and the verb "to continue" is in the present tense. The present tense is used to describe actions that are happening now or that happen regularly. In this case, the speaker is saying that nature is degrading now and will continue to degrade in the future.

The other options are not as appropriate.

• Continue is the infinitive form of the verb "to continue." It is not the correct form to use in this sentence, because it is not describing an action that is happening now or that happens regularly.
• Continued is the past tense form of the verb "to continue." It does not make sense in this sentence, because the speaker is not talking about something that happened in the past.
• Continuing is the present participle form of the verb "to continue." It does not make sense in this sentence, because it is not describing an action that is happening now.