Hello Jitendra

Let's remember the properties of parallelograms

• Opposite angles equal
• Opposite sides equal

And

Rectangle has

• Opposite sides equal
• All angles 90

Now, 

Parallelogram with one angle 90 means

Assume angle A = 90. So, 

• Opposite sides are equal
• Opposite angles are equal,
  • ∠ A = ∠ C = 90 
  • ∠ B = ∠ D = x

Now, in parallelogram,

Sum of angles = 360

∠ A + ∠ B + ∠ C + ∠ D = 360 

90 + x + 90 + x = 360 

 180 + 2x = 360

2x =  360 - 180

2x = 180

x = 180/2

x = 90

So, 

 ∠ A = ∠ B = ∠ C = ∠ D = 90

So, ABCD is a parallelogram with all angles 90

So, ABCD is a rectangle

Thus, parallelogram with one angle 90 is a rectangle.