Nonane (b) has the highest melting point.
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A caveat: I'm assuming that we're dealing with the straight-chain isomers of these alkanes (specifically pentane and nonane). The straight-chain isomer of pentane (n-pentane, CH3-[CH2]3-CH3) has a melting point of -129.8 °C; the straight-chain isomer of nonane (n-nonane, CH3-[CH2]7-CH3) has a melting point of -53.5 °C. The pattern holds as you go down (or up): The more carbon atoms, the higher the melting point. So, in decreasing order of melting points here, you'd have the following: nonane > pentane > butane > ethane.
However, one structural isomer of pentane, neopentane, has a melting point of -16.4 °C, which is higher that the melting point of n-nonane despite neopentane having the same molecular formula as its straight-chain isomer. Of course, you're not to blame for coming up with this question; this is just some extra info to keep in mind.