First use process by elimination. The shaded region is in the fourth quadrant, which means that any potential answer must have negative y coordinate and positive x coordinate. This means that B, C and E can be kicked out immediately.

Now use common sense to show that A is possible and D isn’t. The triangle is 45-45-90 after all, which should give you a hint.

if you see in the quadrant in which the shaded region lies x: has +ve values and y has -ve values, so can eliminate options based on this.

now, the you can check the line expresses the equation y = -x.

and in shaded area if absolute value of x >= absolute value of y it will lie inside the shaded region