ANS:C - H

Neutrons are present in all atoms except for the atoms of Hydrogen (H). Explanation:

• A neutron is one of the three main subatomic particles present in an atom, along with protons and electrons.
• Hydrogen (H) is the only atom among the options listed that does not contain any neutrons in its nucleus. A hydrogen atom consists of only one proton and typically one electron. It does not have any neutrons.
• All other atoms listed (He, C, Ar) have neutrons present in their nuclei. Helium (He), carbon (C), and argon (Ar) all have neutrons in addition to protons in their nuclei.