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The expected return of 12.50% lies between corner portfolios 4 and 5 with expected returns of 12.79% and 10.54%. We solve for w in the following equation:

12.50 = w(12.79) + (1-w)(10.54) 
w = 0.87

In other words, the efficient portfolio with an expected return of 12.50% has 87% weight of corner portfolio 4 and 13% weight of corner portfolio 5. With respect to asset classes, the weights are then derived as follows:

US equity = (0.87)(21.48) + (0.13)(9.40) = 19.91%
US bonds = (0.87)(0) + (0.13)(51.30) = 6.67%
Intl equity = (0.87)(52.01) + (0.13)(26.55) = 48.70%
Intl bonds = (0.87)(5.24) + (0.13)(0) = 4.56%
Alt. invest = (0.87)(21.27) + (0.13)(12.76) = 20.17%

Hence, International bonds are the least significant asset class with the lowest weight of 4.56%.