Which one of the following would you need to identify the gas that evolve when you heat NaOH solution with zinc metal?

Question

$\begin{array}{1 1} \text{(a) Red litmus solution} \\ \text{ (b) Blue litmus solution} \\ \text{ (c) A burning splinter / matchstick} \\ \text{ (d) Lime water} \end{array}$