The correct answer is: in front of.
The preposition “in front of” is used to indicate the location of something that is facing or near the front of something else. In this case, the speaker is giving a speech, which is an activity that is typically done in front of an audience. The preposition “in front of” is therefore the most appropriate choice.
The preposition “of” is used to indicate possession or association. In this case, the speaker is not giving a speech that is owned or associated with the whole class. The preposition “of” is therefore not the correct choice.
The preposition “on” is used to indicate location on a surface or area. In this case, the speaker is not giving a speech that is located on the whole class. The preposition “on” is therefore not the correct choice.
The preposition “none of the above” is used to indicate that none of the other options are correct. In this case, the preposition “none of the above” is not the correct choice, because the preposition “in front of” is the correct choice.