a
Distance between the image (screen) and the object, \(D=90 \,cm\)

Distance between two locations of the convex lens, \(d =20 \,cm\)

Focal length of the lens \(= f\) Focal length is related to \(d\) and \(D\) as:

\(f=\frac{D^{2}-d^{2}}{4 d}\)

\(=\frac{(90)^{2}-(20)^{2}}{4 \times 90}=\frac{770}{36}=21.39 \,cm\)

Therefore, the focal length of the convex lens is \(21.39\, cm\)