"They like the zoo a lot" was not accepted. The answer in English means much the same thing and "mucho" usually means "a lot".
I wrote, "They very much like the zoo." This was marked as wrong. Why?
Sounds fine to me. Please report it (using the flag icon) if you haven't already done so.
If context is clear, it's optional. But without it, there's ambiguity---since les could also mean ellas as well as ustedes.
Les can mean either "they" or "you" (plural), so A ellos/as or A ustedes is often prepended for clarity.
That was probably ¿De verdad? as in "Really?" as in a surprise. Here, really means "a lot."