If you belong to a group of people, you are a member of that group and you are considered part of it. An excellent example is being a member of a Club or translated to Spanish:
Ser miembro de un Club
Therefore, the right answer is D.
The other options, such as:
-Saber (to know)
-Jugar Videojuegos (Playing video games)
-Hacer una búsqueda (to search)
Do not necessarily involve belonging to a group of people.
