Explanation:

An intersection at grade refers to a point where two or more roads meet or cross each other at the same elevation or grade. In other words, it is a junction where vehicles can move from one road to another without the need for any vertical separation or ramps.

There are various types of intersections at grade, each with its own design and characteristics. The options given are different types of intersections at grade, and we need to identify the one that is not an intersection at grade.

Un-channelized Intersection:
- Un-channelized intersection refers to an intersection where no specific lanes or channels are provided for different traffic movements.
- It is a basic and simple form of intersection where vehicles from different directions can cross each other without any guidance or separation.
- This type of intersection is generally used in low-traffic areas or rural areas where the volume of traffic is relatively low.

Channelized Intersection:
- Channelized intersection refers to an intersection where separate channels or lanes are provided for different traffic movements.
- It aims to improve the efficiency and safety of the intersection by guiding vehicles into specific lanes based on their intended movements.
- Channelization can be achieved through the use of pavement markings, traffic islands, medians, and other physical features.

Rotary Intersection:
- A rotary intersection, also known as a roundabout or traffic circle, is a type of intersection where traffic moves in a circular pattern around a central island.
- Vehicles entering the rotary yield to the circulating traffic and merge into the flow.
- Roundabouts are designed to improve traffic flow, reduce congestion, and enhance safety by eliminating high-speed, high-conflict movements.

Different Level Intersections:
- Different level intersections refer to intersections where roads cross each other at different vertical levels or grades.
- This type of intersection may involve the use of ramps, bridges, or underpasses to allow traffic to move smoothly without conflicting with each other.
- Different level intersections are typically used in areas with high traffic volume or where there is a need to separate conflicting traffic movements.

Conclusion:
From the given options, the correct answer is option D - Different level intersections. Unlike the other options, different level intersections do not involve intersections at the same grade or elevation. Instead, they involve vertical separation of roads to allow for smooth and conflict-free traffic movements.