Which equation is perpendicular to y = 2x + 5?

Perpendicular lines have gradients whose product is - 1.

Straight lines in standard form y=mx+c has gradient m and y-intercept c.

So y=2x+5 has gradient 2.

So any line of gradient - 1/2 will be perpendicular to this line, ie any line of from y=-1/2x+c