The correct option is B Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion
Both the assertion and the reason are correct statements. An element which is extremely malleable was required for this experiment to work.
Since gold is the most malleable metal, it was used in the particle scattering experiment. A very, very thin sheet of gold which is just around 1000 atoms thick (thin?) was used in this experiment. That's about 100 times thinner than a human hair!