Read the passage and answer: The bridge was designed to be RIGID so that it would not bend or sway under heavy loads. As used in the passage, RIGID most nearly means:

The context explains the meaning directly: the bridge is RIGID 'so that it would not bend or sway.' Something that does not bend is stiff and unbending. The distractor 'flexible' is the opposite of what the passage describes. Vocabulary-in-context items reward reading the surrounding words: here the phrase 'would not bend or sway' defines RIGID for you. Choose the synonym that matches the passage's usage — stiff and unbending — rather than a meaning the word might have in another context.