Water loss from a leaf, or transpiration, starts within the mesophyll cells where evaporation occurs, and then moves out through the stomata. However, the veins (xylem) are involved in transporting water to the mesophyll, but the initial evaporation happens in the mesophyll. 

Here's the correct order:

2. Veins: The water vapour is transported through the veins (xylem) to the rest of the leaf, including the mesophyll cells.

1. Mesophyll: Water evaporates from the mesophyll cells.

3. Substomatal cavity: The water vapour then enters the substomatal cavity from the mesophyll.

4. Stomata: Finally, the water vapour exits the leaf through the stomata.

So, the correct order is indeed C. 2-1-3-4.