Hey there! I'm happy to help!
Tan means tangent. The tangent is the ratio of the opposite side of the angle to the adjacent side of the angle.
If we look across from angle C, we see that the opposite side is 5.
The adjacent side is the one next to the angle, and it can't be the hypotenuse (the diagonal one). Our adjacent side is 13.
So, the tangent of C is 5/13, and we can divide these, giving us about D. 0.38.
Have a wonderful day! :D