Answer:
Option C is a cubic function
Step-by-step explanation:
Given the options we have to choose the cubic function
Cubic function is the function with degree 3 i.e the highest power of variable is 3.
Option a: 
The above polynomial has highest power 1 i.e of degree 1
∴ not a cubic function
Option b: 
The above polynomial has highest power 2 i.e of degree 2
∴ not a cubic function
Option c: 
The above polynomial has highest power 3 i.e of degree 3
∴ above is a cubic function
Option d: 
The above polynomial has highest power 4 i.e of degree 4
∴ not a cubic function