A(n) usually diagnoses problems with the teeth and gums. a. dental hygienist b. orthodontist c. ophthalmologist d. dentist

Explanation:

Dentists are specialize in dentistry, which is a branch of medicine which consist in the study, diagnosis, prevention and treatment of diseases related with the "mouth" or oral cavity. In other words, they diagnoses problems with the teeth and gums.

On the other hand, orthodontist's job is to fix or correct the position of the teeth, like bad bites or aligning jaws. Basically, they are specialists, which means that they have a higher degree than dentists.

At last, dental hygienist's job is to only prevent oral cavity disease with hygiene treatments, this job is for a sanitarian therapist. And ophthalmologists  treat eyes, they don't specialize in nothing related with teeth.