Answer:
Carbon
Step-by-step explanation:
Valence Electrons
Atoms become ions when they gain or lose electrons, typically to form a full valence shell (full octet). By becoming an ion, they also become charged, becoming either a cation, that is positively-charged, or an anion, that is negatively-charged.
Out of the given options, carbon is the least likely to form an ion. It has 4 valence electrons, meaning to form an ion, it must either lose 4 electrons or gain 4 electrons to have a full valence shell of 8 electrons.
Let's compare carbon to chlorine, for instance. Chlorine has 7 valence electrons, meaning it only needs to gain 1 more electron to form a full valence shell. It's much more motivated and it takes much less energy for chlorine to gain an electron to become an anion.