What does the t stand for in the given equations

Question

Problem:

Subspaces

Determine whether the given sets form subspaces of R². What does the T stand for in these equations?

A. {(x₁,x₂)^T | x1 + x2 = 0 }

B. {(x₁,x₂)^T | x₁x₂ = 0 }

C. {(x₁,x₂)^T | x₁ = 3x_{2 }}

D. {(x₁,x₂)^T | |x₁| = |x₂| }

E. {(x₁,x₂)^T | x²₁ = x²_{2 }}