Your answer would be B, or divergent.
Let's go through what each of these boundaries are.
Transform: When two plates move along their boundary. For example, if you were looking at a boundary running north to south, one plate would be shifting north, the other south.
Divergent: When two plates are moving away from each other, and new land is being made in between them.
Convergent: When two plates are moving towards each other, and land is being destroyed between them.
(Subduction is not a boundary, it has to do with earthquakes but that is a subduction zone, not boundary).
What is being described here sounds to be along the lines of a divergent boundary, as the African plate is separating while moving in opposite directions. This means that the two plates are moving away from each other.
Therefore, I would say the boundary of the East African Rift is a divergent boundary.
Hope this helped!
