[tex]B)90^{\operatorname{\circ}}[/tex]
1) Since this is a square, it is safe to say that there are four right angles in each vertex:
2) Note that when the diagonals cross they form four isosceles triangles:
According to the Triangle Sum Theorem, The sum of all interior angles within a triangle is 180º. So we can find x by doing the following equation:
[tex]\begin{gathered} x+45+45=180 \\ x+90=180 \\ x+90-90=180-90 \\ x=90^{\circ} \end{gathered}[/tex]
