The unexpected loss of the portfolio ($UL_p$) can be calculated as: $UL_p = \\sqrt{n \\times UL_i^2 + n \\times (n - 1) \\times \\rho \\times UL_i^2}$

Where $n = 12$, $UL_i = 85$ million, and $\\rho = 0.3$. $UL_p = \\sqrt{12 \\times 85^2 + 12 \\times 11 \\times 0.3 \\times 85^2}$ $= \\sqrt{86,700 + 286,110}$ $= \\sqrt{372,810} \\approx 610.5817$ million.

The Economic Capital (EC) is the capital multiplier times the unexpected loss of the portfolio: $EC = 7.3 \\times 610.5817 \\text{ million} \\approx 4,457.25 \\text{ million}$, which is approximately JPY 4.457 billion. Thus, Option C is correct.