As Hurricane Hugo approached the Atlantic coast, it increased dramatic

The big question here is whether "the" is necessary. Let's look at choice (A):

"As Hurricane Hugo approached the Atlantic coast, it increased dramatically in strength, becoming the tenth most intense hurricane to hit the United States mainland in the twentieth century and most intense since Camille in 1969."

At first glance, this seems reasonable: "... becoming the 1) tenth most intense hurricane to hit the US mainland in the 20th century and 2) most intense since Camille in 1969." Both items in the parallel list seem to share the article "the", so what's wrong with that?

The problem is that there's a second way to interpret (A): "... becoming the tenth 1) most intense hurricane to hit the US mainland in the 20th century and 2) most intense since Camille in 1969." Was Hugo the 10th most intense since Camille? Or the most intense since Camille? Either could technically work in (A).

The meaning is much clearer in choice (C):

"As Hurricane Hugo approached the Atlantic coast, it increased dramatically in strength, becoming the tenth most intense hurricane to hit the United States mainland in the twentieth century and the most intense since Camille in 1969."

Here there is only one way to interpret the sentence: "... becoming 1) the tenth most intense hurricane to hit the US mainland in the 20th century and 2) the most intense since Camille in 1969."

The meaning is unambiguous in (C), and that's our best choice!