Final answer:
The 1s orbital is closest to the nucleus in the atom of Helium (He).
Step-by-step explanation:
The atom in which the 1s orbital is closest to the nucleus is b) He (Helium).
The 1s orbital is the innermost orbital in an atom and is always filled first before any other orbital. Helium has two electrons, which occupy the 1s orbital, making it the closest to the nucleus.