Bad Qubits — Learn Quantum by Doing

Multiple choice

The lesson states that the modern negative-weight (general) adversary bound ADV-plus-minus(g) has a special status among quantum query lower-bound techniques. What is that status?

It has been proven to always underestimate the true query complexity
It applies only to the OR function and to no other function
It exactly characterizes bounded-error quantum query complexity, up to constants, for every Boolean function
It only ever provides upper bounds on quantum query complexity, never lower bounds