Concept:
A grammatically correct sentence must follow proper rules of subject-verb agreement.
Step 1: Analyze option (A).
``Either'' is singular.
Correct sentence:
Therefore option (A) is incorrect.
Step 2: Analyze option (B).
Rickets is treated as a singular disease.
Correct sentence:
Hence option (B) is incorrect.
Step 3: Analyze option (D).
Bread and butter as a single food item takes a singular verb.
Correct sentence:
Therefore option (D) is incorrect.
Step 4: Analyze option (C).
Every student is singular and correctly takes the singular auxiliary
has.
Thus the sentence is grammatically correct.
Step 5: Conclusion.
Hence, option (C) is correct.