Which merkin? That merkin.
That/which. Save the use of that for information that can’t be removed without changing the meaning of the sentence. For example: merkins that smell bad must be destroyed. By using that, the sentence means that I don’t want to destroy all the merkins in the world, just the ones that smell bad. In contrast, if I say ‘merkins, which smell, must be destroyed’, it means I think they all smell and must all be destroyed. It’s war on merkins. I could also, if I wanted to, leave out the ‘which smell’ and it wouldn’t change the meaning of the sentence: that all merkins must be destroyed. In other words, it’d be a recipe for merkin massacre.