The area of any circle is    (pi) x (radius)² .
The area of the WHOLE circle in the question is
                         (pi) x (2.5 cm)²
                       = (pi) x (6.25 cm²) .
The full circle is made by 360° of central angles.
90° of central angle makes a pie-wedge (sector)
that's exactly one fourth of the circle, so its area
is one fourth of the area of the whole circle.
           Area = (1/4) (pi) (6.25 cm²)
                = (0.25) (3.14) (6.25 cm²)
                =  4.906 cm² .
Rounded to the nearest tenth:  4.9 cm² .
