Answer
The better reducing agents are
Li
$ H_{2(g)}$
$ Fe^{2+}(aq)$
$ Co^{2+}(aq)$
Work Step by Step
The better reducing agents are ones with lower standard reduction potential values.
$Na^{+}(aq)/Na$(-2.71 V) and $Li^{+}(aq)/Li$(-3.05 V)
From the above values, the Li with lower reduction potential (higher negative value) is a better reducing agent.
Comparing $ H^{+}/H_{2(g)}$ [0.00 V] and $ I_{2(g)}/I^{-}$ [+0.53 V], $ H_{2(g)}$ is a better reducing agent.
Comparing $ Fe^{2+}(aq)/Fe_{(s)}$ [-0.44 V] and $ Ag^{+}(aq)/Ag_{(s)} $[ +0.80 V], $ Fe^{2+}(aq) $ is a better reducing agent.
Comparing $ Br_{2(g)}/Br^{-}$ [+1.07 V] and $ Co^{2+}(aq)/Co_{(s)}$ [-0.28 V], $ Co^{2+}(aq)$ is a better reducing agent.
