She _ her parents. They must be worried about her health.

Explanation

The correct answer is Option A: 'She had better call her parents.' The phrase 'had better' is used to give advice or suggest that something should be done. It is always followed by the base form of the verb. Option B is incorrect because it uses the past tense 'called,' which is not used after 'had better.' Option C is incorrect because 'had better' should not be followed by 'to.' Option D is incorrect because 'better' alone does not convey the advice meaning without 'had.'