The building blocks of the universe are called

1)ions
2)elements
3)compounds
4)nuclei

"Nuclei " are called as building blocks of the universe.

Option: D

Explanation:

Smallest unit of this universe was considered to be an atom but “Rutherford 's Nuclear Model of Atom” in 1911 discovered nucleus which is small and heavy positively charged body present in center of atom. An atom also have extra nuclear part which is space around nucleus, where electrons (negative charged) are distributed. Nucleus is contained with protons (positive charged) and neutrons (neutral) which are definitely very smaller than nuclei but as a building unit or block of the universe nuclei is considered.  

the answer would be b i believe