Answer: Asset classes in asset allocation models.

Security market indices are primarily used as proxies for **asset classes in asset allocation models**. This is because they provide a benchmark for the performance of a specific asset class, aiding in the construction and evaluation of diversified portfolios. - **Option A is incorrect**: Nonsystematic risk is specific to individual assets or industries and can be mitigated through diversification. Security market indices, being broad and diversified, do not serve as proxies for nonsystematic risk. - **Option C is incorrect**: Asset-based valuation models are used to value individual companies by estimating the fair value of their assets and liabilities. A broad security market index is not a suitable proxy for such valuations. Thus, the correct answer is **B**, as indices are critical in asset allocation for representing asset classes.

Author: LeetQuiz Editorial Team