For this case we have that by definition, a linear function is of the form

We have the following options:
A)

Rewriting:

It's a quadratic function!
B)

Rewriting:

It's a linear function!
C)

Rewriting:

It's a cubic function.
D)

Rewriting:

It's a quadratic function!
E)

Rewriting:

It's a cubic function!
Answer:
The linear function is option B