A negative correlation between two assets in a portfolio implies that the market value of one asset tends to increase when the market value of the other asset decreases, and vice versa. This inverse relationship can help to reduce the overall risk of the portfolio. If one asset is performing poorly (i.e., its market value is decreasing), the other asset is likely to be performing well (i.e., its market value is increasing). This can help to offset losses from the poorly performing asset and stabilize the portfolio’s overall value. Therefore, a negative correlation between two assets can be beneficial for risk management purposes, as it can help to diversify the portfolio and reduce the potential for large losses.

Choice A is incorrect. A negative correlation between two assets implies that if the market value of one asset decreases, the other asset's value, on average, increases. This reduces the overall risk of the portfolio by providing a hedge against losses from one asset with gains from another. It does not mean that both assets decrease in value simultaneously.

Choice C is incorrect. The statement contradicts itself by suggesting that an increase in one asset's value when another decreases (which is a characteristic of negative correlation) would increase overall risk. In fact, this scenario would reduce overall risk as it provides diversification benefits.

Choice D is incorrect. This describes a positive correlation where both assets move in the same direction together - if one increases, so does the other and vice versa which can potentially increase portfolio risk due to lack of diversification benefits.