Upper option is add doom tokens

Lower option is a monster spawns on each empty monster task

All the adventure cards are facedown, can i choose the lower option?

It'd be helpful if you could add info about the expansion you're referring too...

Its a mythos card from Omens of the Deep.

Then sure, you can choose the second option and resolve it for no effect. From the rules:

"Players may choose either option"

without restrictions of sort, so, you're totally allowed to pick the dud part of the Mythos card