If you are taking an online course and are presenting some problems or have any questions, the most appropriate thing to do <u><em>is D, contact your instructor immediately. (Answer, last option). </em></u>
It is recommended to contact the instructor immediately and only contact him/her because that is the person who can really help you solve questions related to the topics of the course or anything else related to it.
"Today nobody brought his or her instrument" is correct. It is common for people to use "their" in a sentence like this, but "his or her" is the more proper term.
