Complete the sentence: It was ____ a terrible day that we stayed indoors.

• A. So
• B. Such
• C. Very
• D. Really

Answer: (B)

The main idea here is using such to intensify the whole noun phrase “a terrible day” and link it to a consequence with that. Such a + adjective + noun (such as “such a terrible day”) is a classic way to stress just how bad something was and to explain why something followed, in this case that we stayed indoors. The other options don’t fit the exact pattern in the sentence. So cannot pair with “a” in this slot to form the natural “so terrible a day” construction, and very or really would either misplace the emphasis or produce a less natural inflection for this specific structure.