To 'envisage' something means to:

To 'envisage' something means to form a mental image or concept of it, especially something that may happen in the future. The term is often used in contexts where one is visualizing or contemplating potential outcomes, scenarios, or ideas. Envisaging involves thoughtful consideration and imagination, helping individuals to plan or prepare for what lies ahead.

This understanding shows why the other options do not align with the meaning of 'envisage': forgetting past images doesn't capture the active process of forming images about the future; dismissing hypothetical situations contrasts with the idea of considering possibilities; and scheming maliciously has an entirely different connotation, unrelated to mental visualization or conceptualization of future events. Therefore, the focus on mental imagery and the forward-thinking aspect of 'envisage' solidifies the correctness of forming a mental image as the accurate meaning.