Which one of the following is TRUE at any valid state in shift-reduce parsing?

In shift-reduce parsing (bottom-up parsing), a viable prefix is a prefix of a right-sentential form that can appear on the stack of a shift-reduce parser without extending past the right end of the handle [18].

• At any valid configuration of a shift-reduce parser, the string of grammar symbols held in the parsing stack must form a viable prefix of some right-sentential form [18].

• Hence, the parsing stack always contains only a viable prefix (or prefixes) of the grammar at every valid state [18].

Therefore, option C is the correct statement [18].