Three angles are acute angle and each measuring is 75° means

A = B = C = 75°

(Acute angle is angle whose measuring is greater than 0 and less than 90.)

As we know that, Sum of all four angles of quadrilateral is 360^o.

A + B + C + D = 360°

75° + 75° + 75° + D = 360°

D = 360° - (75° + 75° + 75°)

= 360° - 225°

= 135°

So, fourth angle of quadrilateral is 135°.