Half a bracket pair

The loop checks every closing bracket correctly, then returns true. Which input is incorrectly accepted?

for (char c : s.toCharArray()) {
    if (c == '(') stack.push(')');
    else if (stack.isEmpty() || stack.pop() != c) return false;
}
return true;

Assume ordinary supporting code and valid inputs unless the question states otherwise.

Choose an answer before revealing the trace. Keys 1 to 3 work too.