The correct answer is: C. at once.
A jiffy is a very short period of time, so the speaker is saying that they will be back very soon. The other options are not correct because they do not indicate that the speaker will be back very soon.
Option A, in a hurry, could mean that the speaker will be back quickly, but it could also mean that they will be back in a state of excitement or agitation. Option B, by some vehicle, could mean that the speaker will be back using a car, train, or other form of transportation, but it does not indicate that they will be back quickly. Option D, after some time, means that the speaker will be back after a while, which is not the same as saying that they will be back very soon.