The answers are C and E.

Both words 'scarcely' and 'hardly' make a double negative conveying that the potential reduction is hardly inconsequential, or in other words, consequential or important.

A. nearly: "The potential reduction in water lost from the proposed reservoir from evaporation is nearly inconsequential." This implies that the reduction is almost insignificant, emphasizing a minimal impact, which is opposite in meaning to what we are looking for.

B. comparatively: "The potential reduction in water lost from the proposed reservoir from evaporation is comparatively inconsequential." This suggests that, when compared to something else, the reduction is relatively minor. However, it doesn't explicitly convey the idea of the reduction being significant.

C. scarcely: "The potential reduction in water lost from the proposed reservoir from evaporation is scarcely inconsequential." This creates a double negative, suggesting that the reduction is not inconsequential, meaning it is consequential or important.

D. rarely: "The potential reduction in water lost from the proposed reservoir from evaporation is rarely inconsequential." This option introduces an awkward combination, as "rarely" is not typically used in this context. It doesn't effectively convey the intended meaning.

E. hardly: "The potential reduction in water lost from the proposed reservoir from evaporation is hardly inconsequential." Similar to "scarcely," "hardly" introduces a double negative, suggesting that the reduction is significant and consequential.

F. relatively: "The potential reduction in water lost from the proposed reservoir from evaporation is relatively inconsequential." This option implies that, when considered in relation to other factors, the reduction is inconsequential. It doesn't suggest a significant impact.

"Scarcely" and "hardly" can indeed be interpreted as creating a double negative, emphasizing that the reduction is not inconsequential but rather important.