Mapping forward contracts to the spot exchange rate (as well as domestic and foreign interest rates) is the standard and appropriate approach to reduce the number of risk factors in FX forwards. Option B is inappropriate because mapping a corporate bond directly to a government bond ignores credit spread risk. Option C is reversed; typically, coupon-paying bonds are mapped to zero-coupon bonds (cash flows are mapped to standard maturity vertices), not the other way around. Option D is incorrect; a stock market index represents systemic risk, mapping it to a single stock would introduce idiosyncratic risk rather than properly isolating the risk factor.