Who was the first vice president since Martin Van Buren to be elected president?

John Tyler , before president was William Harrison's Vice President.

Sorry, Jenny Roberts. The correct answer is George Herbert Walker Bush. See source.

I didn't bother to look at the source. The correct answer to the question is Theodore Roosevelt, who was McKinley's vice-president.
Roosevelt assumed the Presidency upon McKinley's death, and was elected as President in 1904.
Had the question been, "Who was the first sitting vice president since Martin Van Buren to be elected president," the correct answer would have been George H.W. Bush. But that was not the question.

Jenny, Tyler became President upon Harrison's death, and was never elected to the office.