In fact, (5) medical experts suggested that adults should have been drinking between 12 and 18 cups

The best choice for clarity and tense consistency in the sentence is:

**B medical experts suggest that adults should drink**

So the complete sentence would read: "In fact, medical experts suggest that adults should drink between 12 and 18 cups of water per day based on weight, climate, and exertion levels."

This choice maintains present tense, which aligns well with the ongoing advice provided by medical experts.