A) Just like the flatworm, the human excretory system exists to filter wastes from
--> Incorrect comparison. "flatworm" compared to the "human excretory system"
B) Wastes are filtered in the human excretory system just as in that of the flatworm, from
--> The comparison is blurred here and loses emphasis. There should be a comma before "just" to form a relatively meaningful statement out of this.
C) Like that of the flatworm, the human excretory system exists to filter wastes from
--> Correct comparison and sentence copnstructionD) As with flatworms, the excretory system of humans exists to filter wastes out
--> Incorrect comparison. "flatworm" compared to the "human excretory system"
E) Like a flatworm, a human has excretory systems which exists to filter wastes out of
--> SV error "excretory systems" is used with "exists". Also, which should not be used without a comma when it is referring to the noun before it.