Harlequin ichthyosis, a rare genetic disorder, causes the skin to become thick and scaly. Flaking skin behind the eyelids of individuals with this condition is most likely to damage which structure of the eye

Cornea.

Explanation:

Cornea is the  transparent coat on the colored iris. It helps to focus light on the retina due to its curved nature.Its outer surface is made up of non-keratinized stratified squamous epithelium.It is closest to the eyelid. Therefore, it is more susceptible to damage caused by flaking skin behind the eyelids that occur due to the Harlequin disorder.

"d.) carbon dioxide and hydrogen sulfide"