In the structure of Asset-Backed Securities (ABSs), the senior tranche is typically assigned the highest rating, while the equity tranche is expected to yield the highest returns. This is due to the risk-return trade-off inherent in these securities. The senior tranche is considered the least risky among the three tranches. It is the first to receive cash flows from the underlying assets and is therefore less exposed to default risk. As a result, it is typically assigned the highest credit rating, often AAA. On the other hand, the equity tranche is the most risky. It is the last to receive cash flows and bears the first losses if the underlying assets default. Therefore, it is often unrated. However, to compensate for this higher risk, the equity tranche offers the highest expected returns. This structure allows investors to choose the tranche that best suits their risk tolerance and return expectations.

Choice A is incorrect. While it is true that senior tranches are assigned the highest ratings due to their lower risk, they do not yield the highest returns. The higher returns are typically associated with higher risk, which in this case would be the equity tranches.

Choice B is incorrect. Equity tranches do not receive the highest ratings; instead, they have the highest expected returns due to their higher risk profile. Senior tranches, on the other hand, receive high ratings because of their lower risk but do not offer high expected returns.