Splitting water into its constituent elements releases oxygen, and the bundle sheath cells are the place where rubisco is located in C-4 plants. If we imagine that a non-C-4 plant had bundle sheath cells and split water inside those cells as (A) says, that would still allow oxygen to bind to rubisco and interfere with photosynthesis. So (A) doesn’t offer any advantage. In fact, (A) actually removes the advantage that C-4 plants have, because now the bundle sheath cells don’t protect rubisco from oxygen.

The advantage that C-4 plants have is their ability to keep oxygen from binding to rubisco. If a non-C-4 plant used some other enzyme that performs the same role as rubisco, but which oxygen is unable to bind to, then that plant would have the same advantage as a C-4 plant: oxygen can’t interfere with a key enzyme involved in photosynthesis.

For this to give non-C-4 plants a similar advantage to what C-4 plants have (namely, the ability to keep oxygen from binding with rubisco), we’d need to know that rubisco is located somewhere on the other other side of those impermeable vascular structures, such that oxygen can’t reach it. But the author doesn’t say where rubisco is located in non-C-4 plants, so it’s unclear whether (C) would keep oxygen and rubisco apart. That’s enough to eliminate (C). And even if rubisco were located on the other side of the vascular structures, we’d now have the problem that carbon dioxide gas can’t reach rubisco—but we need carbon dioxide to reach rubisco for photosynthesis to work at all. So either way, (C) presents only problems and no advantage.

The advantage that C-4 plants have is their ability to keep oxygen from binding to rubisco. If a non-C-4 plant split water around the vascular structures of the leaf, as (D) says, then we’d have oxygen released around those vascular structures. But where’s the rubisco in this scenario? (D) gives us no reason to think that oxygen won’t still bind to rubisco and interfere with photosynthesis, so there’s no advantage to be had here.

The role of rubisco is to react with carbon dioxide. (E) wouldn’t offer an advantage with photosynthesis; rather, it would interfere with photosynthesis.