The correct answer is B.

A major risk to crypto investors stemming from the structural aspects of the crypto market is market manipulation and lack of transparency due to unregulated exchanges. The absence of regulation in many parts of the crypto market can lead to unfair trading practices, price manipulation, and a lack of reliable information for investors. These factors can create an uneven playing field, where uninformed or less sophisticated investors are at a significant disadvantage and more vulnerable to losses.

A is incorrect because government-imposed restrictions on crypto trading, while a potential risk, are more about external regulatory actions rather than stemming from the inherent structural aspects of the crypto market.

C is incorrect as decreased liquidity during market downturns, though a valid concern, is a characteristic of many financial markets, not exclusive to or primarily a structural issue of the crypto market.

D is incorrect because over-diversification leading to reduced potential gains is more of a strategy-related risk associated with investment decision-making, rather than a risk arising from the structural nature of the crypto market itself.