Answer:
C.

Step-by-step explanation:
Since both fractions don't have the same denominator, we have to make them with the same denominator before we can actually do the addition.
So, here's the initial problem:

To make them with the same denominator, we'll multiply each side by 1, using the denominator of the other part... like this:

Since we multiply by 1 (expressed differently, but still just 1), the numbers aren't really changed.
Then we make the multiplications, first part:

Now the second part:

We now have both sides of the addition with the same denominator, and we can add the numerators.

We also simplified the denominator at the end, by extracting the 3 which was common.