This is sort of more of an opinionated question I feel, but the answer I would go with would be A. Computer Simulation.
Reasons: B. Equation would maybe help her calculate the rate (speed) at which the clouds are moving, but it does not actually show her the motion of the clouds, which is what she wants to study.
C. Map would allow her a visual representation of countries, not a visual representation of clouds in a thunderstorm.
D. Graph, again, would show her a visual of the rates (speeds) of the clouds moving, but she would not be able to see the actual motion of the clouds.
I say A because it allows her to see a visual diagram of the clouds moving, which she can then track how fast, how often, etc. the clouds are moving in the thunderstorm.
