ABC cannot be used as a benchmark index. A benchmark index is a standard against which the performance of a security, mutual fund or investment strategy can be measured. Hence, an ideal benchmark should be investable, meaning that it is possible to replicate the performance of the index by investing in a portfolio that includes the index components. However, the assets included in the ABC index are all illiquid in nature. Illiquid assets are those that cannot be sold quickly without a substantial loss in value. Exotic Stones, Real Estate, and Art are all examples of such assets. Therefore, it would be difficult to trade the ABC index with ease and it may also require high transaction costs. This makes the ABC index unsuitable to be used as a benchmark index.

Choice A is incorrect. Although real estate makes up a significant proportion of the ABC index (75%), it is not an ideal benchmark for real estate because it also includes other types of assets, namely exotic stones and art. An ideal benchmark for real estate should be composed entirely or predominantly of real estate assets.

Choice B is incorrect. The ABC index cannot be an ideal benchmark for hedge funds as it does not reflect the diverse range of strategies and asset classes that hedge funds typically invest in. Hedge fund benchmarks usually consist of a broad-based market index or a peer group average.

Choice D is incorrect. While the ABC index does include alternative investments such as exotic stones and art, these only make up 25% of the total composition, making it less suitable as a benchmark for alternative investments which should ideally cover a wider range and higher proportion of such assets.