Which of these figures is NOT a quadrilateral?'
A
B
C
D

A

Step-by-step explanation:

Since a quadrilateral has 4 sides and 4 vertices, and in figure A there are only 2 vertices the other 2 are curves and therefore figure A is not a quadrilateral.

Hope this helps!

second option

step-by-step explanation:

the measures of a triangle equal 180. use that to find x.

2x+3x+4x=180

9x=180

9x/9=180/9

x=20

now you know x, you know angle a is 40°, angle b is 60°, angle c is 80°.

because angle a is 40°, the first option doesn't work.

because angle b is 60°, the second option does work.

when you add angle a and b, you get 100°. complementary means it equals 90°, so the third option doesn't work.

when you add angle a and c, you get 120°, which is not 100.