They are all MGF's. The subscript is merely to distinguish them from each other.

You're expected to look up a table of distributions and try to re-write what you've been given in a form that you can recognise.

The first one can be written as .

The second one can be written as .

The third one can be written as .

Big hint: The first two are MGF's of well known discrete distributions. The third is a well known continuous distribution. Note that in each case you will be expected to give the value of the relevant parameters of the distribution.