• AViable prefixes appear only at the bottom of the stack and not inside
• BViable prefixes appear only at the top of the stack and not inside
• The stack contains only a set of viable prefixes
• DThe stack never contains viable prefixes

By definition, at every valid configuration of a shift-reduce (LR) parser, the entire stack content (from bottom to top) is always a viable prefix — a prefix of some right-sentential form that can still be extended to derive a valid string without having to pop past a syntax error. This is the core invariant that makes LR parsing well-defined.