(a) Na
Na is the reducing agent because it loses electrons (gets oxidised from 0 to +1), thereby reducing hydrogen in water from +1 to 0 (H₂).
The reducing agent is the substance that itself gets oxidised (loses electrons) and causes reduction in another substance. Here, Na goes from oxidation state 0 → +1 (oxidised), so Na is the reducing agent. H₂O is the oxidising agent (hydrogen in it is reduced from +1 → 0). Examiners expect you to identify the correct substance and briefly justify using oxidation states or electron transfer.