Which plane extends along the line of sight in elevation?

The correct choice is the slant plane, as it is defined by the line of sight and extends in elevation. In the context of fire control systems and targeting, the slant plane describes the angular relationship between the observer's position and the target, accounting for elevation changes. This plane is essential for ensuring accurate targeting, as it considers both horizontal distance and vertical elevation to calculate the true range and ensure effective engagement of the target.

The slant plane is particularly important in calculating firing solutions, especially in situations where the target is at a different altitude than the observer. By understanding the relationship of the line of sight to elevation, operators can adjust their aiming to account for factors such as gravity drop and atmospheric conditions. Thus, the slant plane serves to bridge the necessary calculations that integrate line of sight and target elevation, which is vital for precise weapon deployment and engagement outcomes.