Which sentence has a formal tone

The correct answer is option D) Scientists have spent years trying to determine how tsunamis can be predicted with greater accuracy.

Explanation:

A formal sentence is devoid of any slang, informal sentence structure, abbreviations or mixed language words.

In the above sentence, proper English vocabulary is used in the sentence. There is no slang words in it. Words are not written in their short forms and neither is any street language used. The sentence is structured in a very professional way and none of the words are misspelled.