The artificer class gets the Infuse Item feature at 2nd level (E:RftLW, p. 57; WGtE, p. 179):
At 2nd level, you gain the ability to imbue mundane items with certain magical infusions. The magic items you create with this feature are effectively prototypes of permanent items.
One of these infusions is called Resistant Armor (E:RftLW, p. 63; WGtE, p. 183):
While wearing this armor, a creature has resistance to one of the following damage types, which you choose when you infuse the item: acid, cold, fire, force, lightning, necrotic, poison, psychic, radiant, or thunder.
The issue lays with the fact that you can only "learn" this infusion once, per the text at the start of the Artificer Infusions list (E:RftLW, p. 61; WGtE, p. 181):
Unless an infusion's description says otherwise, you can't learn an infusion more than once.
Does that mean that each time I use it (after a long rest, since I can only have one Resistant Armor infusion in existence at a time), I can decide which resistance the armor takes on?
Or does it mean that when I learn the infusion, I have to choose a resistance, and can only use that one specific damage type whenever I use the Resistant Armor infusion?
I ask because with a different infusion that has multiple options, Replicate Magic Item (E:RftLW, p. 63; WGtE, p. 182), the wording is:
Using this infusion, you replicate a particular magic item. You can learn this infusion multiple times; each time you do so, choose a magic item that you can make with it, picking from the Replicable Items tables below.
I understand that specific beats general, which leads me to believe that Resistant Armor allows the caster to choose the resistance (and change it after a long rest).