An object is 20 cm in front of a concave mirror with 10 cm focus. If the height of the object is 1 cm, so...

1 / f = 1 / s + 1 / s'
s' = sf / (s - f)
    = (20 cm)(10 cm) / (20 cm - 10 cm) = 20 cm
Distance of shadow is 20 cm

M = -s' / s
    = -20 cm / (20 cm) = -1 
Shadow magnification of 1 times

M = -s' / s = h' / h
h' = Mh = -1(1 cm) - 1 cm
Height of shadow is 1 cm

The answers is D.