The nucleus of an atom is
a. the central core and is composed of protons and neutrons.
b. positively charged and has more protons than neutrons.
c. negatively charged and has a high density.
d. negatively charged and has a low density.

a. The central core and is composed of protons and neutrons.  

Explanation:

A nucleus of anything is a central core. The nucleus of an atom is composed of protons and neutrons.

b. is wrong. Except for H, the number of neutrons is always equal to or greater than the number of protons.

c. and d. are wrong. The nucleus is positively charged.

i'm assuming that the correct answer is indeed the first one/ "a"..