Final answer:
The correct word in the sentence is 'portends', which means to indicate or predict that something, usually something bad, is about to happen, such as an oncoming storm.
Step-by-step explanation:
The correct word to use in the sentence "A plunge in atmospheric pressure (conciliates, portends) an oncoming storm." is portends. To portend means to indicate that something is likely to happen, especially something bad. When atmospheric pressure drops sharply, it usually signals that a storm may be approaching. This is because low atmospheric pressure typically leads to the formation of weather fronts that cause stormy conditions. The phrase 'a plunge in atmospheric pressure portends an oncoming storm' therefore suggests that the drop in pressure is a warning sign of an impending storm, potentially with effects such as torrential rainfall and high winds.