Because despite being natural they still harm the ozone layer and cause breathing problems, even suffocation.
The question can be changed into a new form:
Which element has the most negative electron affinity, or attraction for electrons? halogens have the highest electron affinities, and thus are more attracted to the electrons in the Hydrogen atom than any element in their respective periods.
In this case all the following choices are in the same period, thus Cl or Chlorine is the answer as it is a halogen.
I think this is learned in chemistry do you have any notes that can help
