Which expression gives the needed margin as a percentage of capital?

Which expression gives the needed margin as a percentage of capital?

• A. Investment yield x corporate tax rate / (1 - corporate tax rate) divided by sqrt(1 + investment yield)
• B. Investment yield x corporate tax rate x (1 - personal tax rate)
• C. Investment yield x corporate tax rate / (1 - corporate tax rate)
• D. Investment yield x corporate tax rate / (1 - corporate tax rate) divided by (1 + investment yield)

Answer: (A)