The correct sentence that is written correctly is: A) Ever since I was a teenager, I have loved going to the theater.
Here are why the other options are wrong:
B) 'although the lasagna looked terrible, it tasted wonderful.' <-- "although" is not capitalized at the beginning of the sentence. If it's the start of a sentence, you have to capitalize the first letter of the first word.
C) 'Pepper and i decided it was time to head home.' <-- The issue with this is that the sentence does not capitalize "I". Nobody really knows why "I" needs to be capitalized since it is a first person pronoun.
D) 'while Jonathan was napping, I finished doing the dishes.' <-- "While" is not capitalized at the beginning of the sentence, making it improper.