No, here Prefix closed means for every string x in L, all its prefixes should also be in the language.
let say x = [ [ ] ] ( x is in L)
'[' is its prefix but it is not in L, So L isn't prefix closed.
In other way, epsilon is prefix of every string but this grammar doesn't generate epsilon So it can't be prefix closed.
Don't go by the prefix property here, read the definition of prefix closed in the question and solve accordingly.