Assertion (A) is true because Rutherford concluded from his gold foil experiment that the nucleus is dense and contains most of the mass of an atom. Reason (R) is also true as Thomson's model proposed the atom as a sphere of uniformly distributed positive charge with electrons embedded in it (like plums in a pudding). However, Reason (R) does not explain Assertion (A) because Rutherford's conclusion about the concentrated mass at the nucleus was based on his own experiment and actually contradicted Thomson's model of uniformly distributed charge. Therefore, both A and R are true, but R is not the correct explanation of A.