Speak Good English: Get it correct

Last week while explaining the correct answer to question 5 of our objectives test, we went into the topic concord and the basic principles underlying it.

We said a singular subject agrees with a singular verb, while a plural subject agrees with a plural verb.

Note that in English, singular verbs have the letter -s or the letters -es after the simple verbs, while plural verbs don't have any such letter or letters after the simple verbs.

Egs:
The boy does his work very well.
The boys do their work very well.
She comes to school every day.
They come to school every day.
Now back to the test.
Question 6 reads:

All but the new student --- the headmaster.
a) knows  b) is knowing  c) is known  d) know

This question too concerns concord  --- subject/verb agreement.

The subject of the sentence is actually ALL, and not STUDENT, but the nature of the sentence may deceive you to go for a singular verb beca