Answer:
Call 911.
Step-by-step explanation:
Marissa should call 911 because her partner is presenting symptoms about an unknown reason, her nauseous feelings most be because of the lack of fluids, but Marissa doesn't exactly know the cause, that's why she can just ask for an intravenous (IV). The best decision here is to call professional aid, which can be provided by calling 911, they will diagnose what's the health issue here and how should be treated.
On the other hand, other options are just wrong, they don't make sense because they will worsen the problem. By staying in the Sun or adding more clothes, Marissa could get her partner more nauseous, and more dehydrated.