Option (a) ⇒ usual form of abc is 100xa + 10xb + c.

Option (b) ⇒ The usual form of abco is 1000xa + 100xb + 10xc + 0x1 = 1000 a + 100 b + 10c. Therefore (b) is not correct option

Option (c) ⇒ The usual form of aobc is 1000xa + 100xo + 10xb + 1xc = 1000 a + 10 b + c. Therefore option (c) is correct.

Option (d) ⇒ The usual form of aboc is 1000xa + 100xb + 10xo + 1xc = 1000 a + 100 b + c. Therefore option (d) is incorrect.