Step 1: Understand the task.
We must find the one sentence that is grammatically correct. The test is mostly about subject-verb agreement.
Step 2: Check option A.
'Either of them have to come' is wrong. 'Either' is singular, so it needs 'has', giving 'Either of them has to come.'
Step 3: Check option B.
'Rickets are easy to cure' is wrong. 'Rickets' is a single disease and takes a singular verb, so it should be 'Rickets is easy to cure.'
Step 4: Check option D.
'Bread and Butter are his food' is wrong. 'Bread and butter' is taken as one food item, so it needs a singular verb, 'Bread and butter is his food.'
Step 5: Check option C.
'Every student has attended the meeting' is correct. 'Every student' is singular, and 'has attended' is the matching singular verb.
Step 6: State the conclusion.
The correct sentence is 'Every student has attended the meeting.'
\[ \boxed{\text{Every student has attended the meeting.}} \]