Is it possible to add a direct object like "le" or "la" before voir or most we absolutely use ca and cela?
When you are given "this/that", generally you are expected to translate "ce/ceci/cela/cet/cette/ces".
However, you can indeed use "le/la" before voir: "c'est impossible de LE/LA voir à l'oeil nu" (it is impossible to see IT to the naked eye)
an option given is the impersonal expression: oui il est impossible de voir ça à l'oeil nu; however it would not accept cela instead of ça, what gives?
Duolingo may not give all correct alternatives, but on principle, since "ça" is the familiar version of "cela", you can always use "cela".