X and $\sigma _{x}$ are the mean daily demand and standard deviation of daily time
L and $\sigma _{L}$ are the mean lead time in days and standard deviation of lead time
Which one of the following is the correct expression for standard deviation of total demand
i.e., lead time demand
(a) $\sqrt{x^{2}\sigma_{L}^{2} + L^{2}\sigma_{x}^{2} + \sigma_{x}^{2}\sigma_{L}^{2}}$
(b) $\sqrt{x^{2}\sigma_{x}^{2} + L^{2}\sigma_{L}^{2} + \sigma_{x}^{2}\sigma_{L}^{2}}$
(c) $x\sigma _{L} + L\sigma _{x} + \sigma _{x}\sigma _{L}$
(d) $x\sigma _{x} + L\sigma _{L} + \sigma _{x}\sigma _{L}$