which scientist discovered the nucleus?

The discovery of the nucleus depends on what you mean: the nucleus of a cell or the nucleus of an atom.

• The cell nucleus was discovered and named by the Scottish botanist Robert Brown in 1831–1833 while studying plant cells.

• The atomic nucleus was discovered by the physicist Ernest Rutherford in 1911 through his famous gold‑foil (alpha scattering) experiment.

If your question is from biology , the expected answer is Robert Brown ; if it is from physics/chemistry , the answer is Ernest Rutherford.