Answer:
Lithium
Step-by-step explanation:
Although sodium may seem like a viable answer because it has more protons (and thus a greater attractive force) in its nucleus, we need to take into account what "valence electrons" means. Valence electrons are, simply, electrons that are on the outermost electron shell of an atom. Sodium has an additional shell of electrons compared to lithium that are far away from the nucleus. Thus, lithium, which has valence electrons closer to its nucleus, would have a stronger attraction between its nucleus and its valence electrons.