Out of the given four options which one does not belong to the class of other…

Classification (odd-one-out) questions test whether you can spot the single category that binds three items together, so that the item lacking it is the answer. The shared category can rest on meaning (what the words denote) or on structure (a common suffix/prefix), and checking both gives a more certain result.

Here, three of the four terms name licensed medical specialists — each a doctor certified in one branch of modern medicine:

• Neurologist — treats disorders of the nervous system.

• Cardiologist — treats disorders of the heart.

• Gynecologist — treats disorders of the female reproductive system.

Alchemist names a practitioner of alchemy, a pre-modern practice of experimenting on substances (most famously trying to turn base metals into gold). It is not a branch of modern medicine, and an alchemist holds no medical qualification, so this term does not share the medical-specialist category the other three share.

Cross-check at the word-formation level: Neurologist, Cardiologist and Gynecologist all end in the suffix "-ologist" (a specialist in an "-ology", i.e. a field of study), while Alchemist ends in "-chemist"/"-mist", not "-ologist" — an independent structural check that agrees with the meaning-based grouping.

So the option that does not belong to the class of the other three is Alchemist.