Answer:
A. Two continental plates moving away from each other, forming a rift valley
Step-by-step explanation:
The google description is:
A tectonic boundary where two plates are moving away from each other and new crust is forming from magma that rises to the Earth's surface between the two plates.
Based on that you can infer that A. Two continental plates moving away from each other, forming a rift valley
is the best answer.
though getting a second opinion might be best cause I am no expert in this subject.