Walnut.

Thanks, how do you differentiate walnut from hickory at this size?

"how do you differentiate walnut from hickory at this size?"

It's done by familiarity from previous experience, in the case of young seedlings like this. The two hickories you mention have fewer leaflets per leaf; some other hickories (Pecan, Bitternut) do have as many leaflets, but have a different leaflet shape.

When the tree is older and large enough to spare a twig being snipped off, a 100% reliable way to tell is cut a shoot lengthways; walnuts have chambered pith, while hickories have solid pith.

Resin

what resin said.
Even in that small photo, the leaves, new apical growth and buds at the leaf axils shout 'walnut' to me.

Additionally, you could gently probe down at the base of the seedling and find the nut from which it is growing - providing conclusive proof of what it is.

Yup, definitely a walnut. I have black walnuts as well as both a shellbark and a shagbark hickory, and I pull seedlings by the scores every year, often in large clusters where squirrels buried a cache of nuts and they all sprouted.

With a little experience, you can learn to differentiate them pretty easily. As a rule, they are different enough to be able to do it by sight. Every once in a while a really small seedling is a bit ambiguous.

One really good way, though, to tell the difference is to crush some of the leaves and smell them -- hickory and walnut each have a distinctive aroma, and once you know it, you won't forget it. This "smell test" won't differentiate between the two hickory species, though, as they smell alike.