Option C is the correct answer.
Option A is incorrect because the phrase "in once" is not grammatically correct. The correct expression would be "at once" to mean immediately.
Option B is wrong due to the incorrect use of the article "a" before the word "anxious," which starts with a vowel sound—so it should be "an anxious hour."
Option D is a distorted version of the common idiom; the standard form is "A drowning man catches at a straw", not just "catches straw," which makes the sentence incomplete and incorrect.
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