Rutherford's scattering experiment, conducted by Ernest Rutherford in the early 20th century, is crucial for understanding the structure of the atom. In this experiment, Rutherford and his team directed a beam of alpha particles (which are positively charged particles) at a thin sheet of gold foil and observed the pattern of scattering of these alpha particles after they hit the foil. The observations were unexpected at the time. Most of the alpha particles passed straight through the foil with little or no deflection, but a small fraction of the particles was deflected at very large angles, and some were even scattered back in the direction they came from.

This experimental evidence led to the conclusion that most of the atom is empty space, and all the positive charge and almost all the mass must be concentrated in a very small central region within the atom. This dense central region was named the nucleus. Hence, Rutherford's scattering experiment is directly related to discovering the size and presence of the nucleus in the center of atoms. Therefore, the correct answer is:

Option A: nucleus