Question

D = √[( x₂ - x₁) + (y₂ - y₁)]

Answer 1

As it stands, it's none of those. I believe you were going for the distance formula but copied it wrong.

Answer 2

$It's\ none\ of\ those.$

$d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}\Leftarrow Distance\ Formula\\\\A(x_1;\ y_1);\ B(x_2;\ y_2)\Rightarrow d=|AB|$