Correct Answer: D
Explanation:
There are two problems with the original sentence.
(1) The subject (The Battle of Bull Run) lacks a verb. was or is must be inserted so that the subject can have a verb.
(2) The phrase in that this historic event was is wordy and could be made more concise.
A. the subject (The Battle of Bull Run) lacks a verb; the phrase in that this historic event was is wordy and could be made more concise
B. the subject (The Battle of Bull Run) lacks a verb; the phrase event in American history marking the first major ground needs to be split up by a comma after history in order to give the correct idea that marking modifies the idea that precedes it
C. the past tense was gives the false impression that the Battle was at some point in the past important, but it is not important today; the past perfect tense had markedis incorrect
D. the subject has a present tense verb; the sentence is concise; the , which modifies what directly precedes it (namely the Civil War) and this modification is logical
E. the phrase resulting in the death of over 500,000 illogically modifies the subject that precedes it (The Battle of Bull Run), giving the false impression that over 500,000 people died in this one battle
Note: which modifies the word or term immediately preceding the comma (i.e., the Civil War) in the correct answer while resulting in the death of over 500,000 modifies the subject or idea to which it refers (The Battle of Bull Run)