C, D, E
You have a triangle with 3 sides labeled S, E, O and 3 vertices labeled D, K, M. To name a triangle, what you do is simply list all of its vertices. The order doesn't matter. So let's take a look at all the possible choices.
A. SKD - Nope.K and D are OK. But S is the line segment.
B. KE - Nope. K is OK, but E is the line segment.
C. MDK - Yes. All three vertices have been named. This is an acceptable name for the triangle.
D. DKM - Yes. All three vertices have been named. This is an acceptable name for the triangle.
E. KMD - Yes. All three vertices have been named. This is an acceptable name for the triangle.
F. SKMO - Nope. Only has 2 of the vertices and S and O are line segments.