(c) 40 cm
For magnification $m = -1$, we need $v/u = -1$, so $v = -u$. Using lens formula with $f = 20$ cm and $u = -40$ cm gives $v = +40$ cm. ✓
Magnification $m = v/u = -1$ means the image is real, inverted, and same size — object must be at $2f$. Since $f = 20$ cm, object distance $= 2 \times 20 = 40$ cm. According to Table 9.4, when object is at $2F_1$, image is at $2F_2$, same size, real and inverted ($m = -1$). Option (c) is correct.