(A) The trail is 6.3 miles long and was fairly easy, but toward the top,it became steep and rocky.
Explanation:
Transitional words are words (there are also phrases) that are used when the author wishes to connect two different ideas. These words serve as a bridge between these ideas, making the reading pleasant and cohesive. These words also allow the reader to realize that different things (such as tenses, or different words) are connected and establish a unique meaning.
Among the options given in the question, we can see that the letter A shows an example where the use of a transitional word has been efficiently established to relate two different verb tenses.
But I will answer your question if you answer mine. Do you mean to write a letter for teens to read (hypothetically)? As in " Dear teens, You need to take a look in the mirror and note all of the good things about yourself......"?