Answer:
C. 
Step-by-step explanation:
We have the arithmetic sequence 15,11,7,3,-1,...
Where,

You can see that the common difference d=(-4), or if you couldn't see the common difference you can calculate it with the <em>formula:</em>

Then,

Now to find the 30th term of the arithmetic sequence we can use the following <em>formula:</em>

Replacing n=30,
and d=(-4):

Then the correct option is C. 