We require one word that can fit naturally into all four blanks while maintaining correct meaning in each sentence.
In sentence A — "People sensed ............." — "disaster" fits perfectly to convey a sense of impending danger or catastrophe.
In sentence B — "A bad ............. case had come in – a person with a smashed arm." — "disaster" works figuratively here to describe a severe accident or mishap leading to injury, whereas other options like "accident" might fit but would not be as flexible across all sentences.
In sentence C — "And then, without warning, ............. struck." — "disaster" is the most natural choice to describe an unexpected and destructive event. It is commonly used in this exact form: "disaster struck."
In sentence D — "The dogs were the first to recognize the signs of oncoming ............." — "disaster" again fits naturally, implying they sensed a major crisis or catastrophic event approaching.
"Accident" is too specific to unplanned mishaps, "tragedy" focuses too much on emotional loss, and "calamity" is more literary but less common in conversational and descriptive use.
Therefore, "disaster" is the only choice that fits smoothly and meaningfully in all four cases.