Ans C.
The following statement is true at any valid state in shift-reduce parsing:
The stack contains only a set of viable prefixes
In shift-reduce parsing, the stack is used to store a set of viable prefixes of the input string, which are sequences of terminals and non-terminals that can potentially form a valid sentence in the grammar. At any valid state in the parsing process, the stack contains a set of viable prefixes that have been generated so far, and the top of the stack represents the currently active prefix. The parser uses a set of shift and reduce actions to transform the input and stack, until the entire input string has been processed and a complete sentence is formed on the stack.
The other statements are not necessarily true, as viable prefixes can appear inside the stack as well, not only at the top or bottom. The stack may contain not only a set of viable prefixes but also non-viable prefixes.