The Splatoon 2 special weapon that gives temporary armor is Ink Armor.
Ink Armor is a team-support special weapon introduced in Splatoon 2. When activated, it applies a protective effect to the user and teammates, allowing them to withstand damage that would normally splat them. The protection is temporary and is not the same as permanent health or an invincibility state.
Splatoon’s special weapons charge as players cover ground with their team’s ink. Once ready, a special can dramatically change the battle: some attack directly, while others provide mobility, area control, or support. Ink Armor belongs to the support category because its main value comes from strengthening the whole team at an important moment.
The other options are genuine Splatoon weapons, which makes this question easy to confuse. Sting Ray is a long-range beam weapon, Bubble Blower creates protective bubbles, and Splashdown is a powerful area attack. Bubble Blower can provide cover, but it is not the special specifically named Ink Armor.