Concept:The sentence uses two facts that go against each other: Mary tried hard, yet she did not succeed.
Explanation:The first part of the sentence says that Mary did her best.
The second part says that she still failed to reach the pass mark.
These two ideas are opposite in meaning, so we need a word that shows contrast.
The coordinating conjunction
but is used to join two contrasting clauses.
If we choose “however,” the sentence would need a semicolon or a full stop before it, not just a comma.
The word “so” shows a result, not a contrast.
The word “and” merely adds information without showing any opposition.
Therefore, the only correct option that fits the structure and meaning is “but.”
Answer:A. but