Diffusion current is a type of current that arises due to the movement of charge carriers from regions of high concentration to regions of low concentration. It is associated with the diffusion process, where charge carriers move in response to concentration gradients.

The diffusion current is primarily observed in a semiconductor device such as a transistor, where the movement of charge carriers plays a crucial role. In the case of a transistor, there are three regions: the emitter, the base, and the collector.

Let's analyze each option to determine which ones are associated with diffusion current:

a) Base current: The base current is the current that flows into the base terminal of a transistor. It is responsible for controlling the current flow through the emitter and collector terminals. The base current consists of two components: the diffusion current and the drift current. The diffusion current arises due to the movement of charge carriers from the emitter region to the base region. Therefore, the base current is associated with diffusion current.

b) Emitter-base junction current: The emitter-base junction current is the current that flows across the junction between the emitter and the base of a transistor. In an unbiased transistor, this current is primarily due to the diffusion of charge carriers from the emitter region to the base region. Hence, the emitter-base junction current is associated with diffusion current.

c) Emitter-base current: The emitter-base current is the sum of the base current and the collector current in a transistor. It represents the total current flowing into the emitter terminal. While the base current contains a diffusion current component, the emitter-base current is the sum of the diffusion current and the drift current. Therefore, it is not solely associated with diffusion current.

d) Base-collector junction current: The base-collector junction current is the current that flows across the junction between the base and the collector of a transistor. In an unbiased transistor, this current is primarily due to the diffusion of charge carriers from the base region to the collector region. Hence, the base-collector junction current is associated with diffusion current.

In conclusion, the options associated with diffusion current are:
- Emitter-base junction current (Option B)
- Base-collector junction current (Option D)