The text for Fire bolt states:

You hurl a mote of fire at a creature or object within range. Make a ranged spell Attack against the target. On a hit, the target takes 1d10 fire damage. A flammable object hit by this spell ignites if it isn’t being worn or carried.

This spell’s damage increases by 1d10 when you reach 5th level (2d10), 11th level (3d10), and 17th level (4d10).

Which level does this use? The spellcasting level or the player level?

For a specific example: I have a player who is Bard 1/Druid 4, who, at Druid 4, taken the Magic Initiate feat:

Choose a class: bard, cleric, druid, sorcerer, warlock, or wizard. You learn two cantrips of your choice from that class’s spell list.

• In addition, choose one 1st-level spell from that same list. You learn that spell and can cast it at its lowest level. Once you cast it, you must finish a long rest before you can cast it again using this feat.
• Your spellcasting ability for these spells depends on the class you chose: Charisma for bard, sorcerer, or warlock; Wisdom for cleric or druid: or Intelligence for wizard.

And has chosen Fire bolt.

Do they cast this at 5th level?