Answer: D) He wants the dogs to go.
The website etymonline says the following:
mush (interj.)
command to sled dogs, 1897, first recorded 1862, as mouche, perhaps altered from French marchons! "advance!" (imperative of marcher "to march;" see march (v.))
So basically the driver is shouting "march!" (more or less) to the dogs for them to move forward. The "interj." means "interjection".
