In the following question, out of the four alternatives, select the alternative which is the best substitute for the given words/sentence.
Have saliva dripping copiously from the mouth

Let's check the definitions:
Slobber: Have saliva dripping copiously from the mouth; drool.

Cascade: A small waterfall, or a process where something pours down in large amounts.

Inundate: Overwhelm someone with things; flood.

Sluice: Wash or rinse with a shower or stream of water.

Therefore, Slobber is the correct substitution.