Option 1: This option states that water gets concentrated on the inner bank of the turn, making it denser. While it is true that water may accumulate on the inner bank, making it denser, this does not necessarily mean that it is the primary reason for the riverbank being weakest at the turn.

Option 2: This option explains that water effectively bounces off the outer bank as it turns, exerting extra pressure on the bank. This increase in pressure can weaken the bank over time, making it more susceptible to erosion. This is consistent with the concept of hydrodynamics and the principles of fluid mechanics.

Option 3: This option suggests that water flows faster as it turns. While it is true that the velocity of water may increase as it navigates a turn, this alone does not explain why the riverbank is weakest at the turn.

Option 4: This option states that water reacts more effectively with the bank at a turn. However, it does not provide a clear explanation for how or why this increased effectiveness would weaken the riverbank.

Therefore, the correct answer is option 2, as it provides a logical explanation for why the riverbank is weakest at the turn by considering the extra pressure exerted on the bank as water bounces off