The formerly ____ waters of the lake have been polluted so that the fish are…

Concept: A sentence-completion vocabulary question asks for the word that fits the logical relationship the sentence sets up -- most often a contrast signalled by markers such as ‘formerly’, ‘once’, or ‘used to be’ against a described present state. The correct word must describe the earlier condition in a way that logically explains the stated effect.

Application: The sentence says the lake’s waters were ‘formerly ____’ but have since ‘been polluted so that the fish are no longer visible from the surface.’ ‘Formerly’ signals that the fish were visible from the surface before the pollution, which requires the water to have been transparent enough to see through -- exactly what ‘pellucid’ (extremely clear, letting light and sight pass through) describes.

Cross-check: Testing each option against the required contrast confirms the choice:

• muddy describes a turbid, sediment-clouded state -- this matches the CURRENT polluted condition, not the clearer state the sentence contrasts it with.

• tranquil describes calm, undisturbed motion, not optical clarity; water can be tranquil and still completely opaque, so it does not explain how the fish were once visible.

• stagnant describes non-flowing, often foul water -- again a condition tied to decline, not the transparency the earlier state requires.

• pellucid alone denotes water clear enough to see through, which is exactly why the fish were visible before the pollution set in.