The italicized word (magnitude) in the sentence can best be matched by the word: size. This is referring to the size of the disaster, that it is hard to imagine something happening on that size scale.
The answer that best matches the word in italics is size.
Explanation:
The sentence "The MAGNITUDE of the disaster is hard to imagine" refers to the amount of damage and consequences that the disaster had let in the land and it's population, in this context the meaning would be size as a way of measuring the quantity in numbers that affected different areas of the place where the disaster happened
