Answer:
A. Almost all of the mass is concentrated in the nucleus.
Step-by-step explanation:
Because when he shot the alpha particles towards the atoms, most passed through (which meant atom is mostly empty space), but some bounced back and such (which meant mass is concentrated in the nucleus.)
The reason they bounced is because alpha particles are postive and nucleus is positive as well, and we know positives don't attract, rather they repel thus they bounced.