They have ............... their lessons for life.

Concept

The present perfect simple is formed with have/has followed by the past participle of the main verb. The auxiliary marks the tense, while the main verb supplies its third form.

Application

1. The subject is they, and the sentence already provides the auxiliary have.

2. After have in the present perfect simple, the main verb must be a past participle.

3. The past-participle form among the offered values is learned, so the completed sentence is ‘They have learned their lessons for life.’

Contrast

• learn is the bare form used after infinitival to or modal auxiliaries, not the third form required here.

• learning is the present participle used in continuous constructions with a form of be.

• learns is the third-person singular simple-present form used with subjects such as he, she, or it.

Cross-check

Substituting learned gives the standard pattern they + have + past participle, so the verb phrase is grammatically complete.

Therefore, learned completes the sentence.