An essential question is a question that provokes thought and sparks more questions. It can't be answered in one sentence.
Let's look at the options one by one, starting from the last.
D. On which continent is the world's tallest building located?
This question can be answered in one word, really. It only asks for the name of the continent. It is not an essential question.
C. What percentage of earthquakes actually cause damage each year?
This question also can be answered in just one sentence. It only asks for a percentage. It also is not an essential question.
B. In which year did the civil war begin, and when did it end?
Again, this question can be answered by simply giving the years when the civil war began and ended. It is not an essential question.
A. What are the similarities and differences between Earth and Mars?
Can this question be answered in one sentence? No. There are many similarities and differences between the two planets, and this question provokes thought. It cannot be answered by simply providing a percentage, or date. To answer the question, you need to analyze how the planets are alike. It is an essential question.
Therefore, the answer is A.
