Question

All sentences below are wrong except:

P.S.: Could you please explain to me your answer? And, why not I could choice the others alternatives?

a) I must to go now. I have an appointment with my doctor
b) Do you may help me? I didn’t understand that explanation
c) She ought to study harder if she really wants to pass the exam
d) They might not to travel on December. They are still thinking about it

Answer 1

The correct sentence is "c"....

A: You can´t use "to" after must....  the correct: I must go now....

B: You can´t use "2 auxiliary verbs" in the same phrase, in this case "do" and "may" .... The correct: May you help me?  

C: correct :D (ought to = advice)

D: We don´t use "to" after might .... the correct: They might not travel (...) ....

[And I think is IN December, not On december]