Translation:The whole cook is wet because he is doing the dishes too fast.
That was my preferred translation, "the cook is all wet." In my dialect (American English, West Coast) it does not mean grumpy, just wet.
I was trying to guess where you are from and I came across a definition of a "wet blanket" for a grouch in the Oxford dictionary, that was as close as I found.
Btw, since we all agree that nobody would say "the whole cook is wet," we can help improve the course by reporting it. Hit the report button which is on the lesson page in the lower right.
In my case I had word tiles so I got the answer correct, and there was no possibility of reporting "the English translation is wrong," so I had to put "something else went wrong."
Interesting, never heard wet = grumpy / dour. If i were all wet i'd be grumpy though
"whole cook" is bad. You wouldn't say that part of the cook is wet. The cook is soaked, drenched, soaking wet, all wet (depending on context, since it can also be an idiom) or completely wet etc. But in this course, you can't say something as you would normally say it, because if you do, it's wrong, even if you're saying the same thing.
"The cook is all wet" is correct, but not "The cook is whole wet". Whole can only be used as a quantifier with singular nouns, not with adjectives. Or can be used as an adjective with plural nouns, meaning "complete" (as in whole grapes are used to make wine), that wouldn't work here, and neither would "whole" as a noun.