All of the following are ways to induce an emf in a loop of wire except which one? a. move the loop into or out of a magnetic field. b. rotate the loop in a magnetic field. c. move the loop parallel to a magnetic field. d. insert the loop into a changing magnetic field.

Faraday's law of induction states the following:

The strength of the induced electromotive force is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit.
Knowing all this, we can conclude that the answer is C. 
Flux can be understood as the number of field lines interesting with the area of the loop. You can simply think that stronger field has more field lines. This is just a useful simplification.
When you simply move the loop along the lines of magnetic field there is no change in flux. If you are rotating or moving the loop in and out of the field you are changing the are of the loop that intersects with the field and thus you are changing the flux. The same thing happens when you have a changing field, the only difference being that field is doing all the work for you.