An atom of the most common isotope of neutral potassium has a nucleus composed of 19 what

If you were to look at a periodic table, you'd see that potassium (K) has an atomic number of 19, which is how many protons it has. Also because it's neutral, the number of protons = the number of electrons, but only protons and neutrons are inside the nucleus.

So the answer would be that the nucleus is composed of 19 protons.