Let's break down the given problem related to the isobaric ternary liquidus projection diagram.

An isobaric ternary liquidus projection diagram helps in understanding the phase relationships at constant pressure. Here, the diagram shows solid phases A, B, C, D, and E in equilibrium with the liquid.

The objective is to determine the correct reaction occurring at the isobaric invariant point W.

Let's analyze the options step-by-step:

1. Liquid (at W) = B + D + E: This suggests that at point W, the liquid phase is coexisting with solid phases B, D, and E. However, this scenario is not consistent with the ternary equilibria shown, where reactions typically involve the transformation of phases.
2. Liquid (at W) = A + B + D: Similar reasoning applies here; this option suggests coexistence rather than a reaction, which doesn't typically indicate a phase transformation at an invariant point.
3. Liquid (at W) + E = B + D: This represents a peritectic reaction where the liquid phase and solid phase E produce solid phases B and D. This is consistent with peritectic reactions found in such diagrams, where the liquid reacts with a solid to form new solid phases.
4. Liquid (at W) + B + D = E: This would imply that B and D are reacting with the liquid to produce phase E, which doesn't align with the concept of an invariant reaction in this context.

Therefore, the correct reaction taking place at the isobaric invariant point W is Liquid (at W) + E = B + D, indicating a peritectic reaction. This scenario matches the typical phase transformations depicted in ternary phase diagrams.