A portfolio consists of two securities: a 90-day T-bill and the S&P/TSX Composite. The expected return on the T-bill is 4.5 percent. The expected return on the S&P/TSX Composite is 12 percent with a standard deviation of 20 percent. What is the portfolio standard deviation if the expected return for this portfolio is 15 percent?

a) 8.13%

b) 12.00%

c) 16.80%

d) 28.00%