Final answer:
Sam spent an extra 1/2 hour on his history assignment more than he did on his English assignment.
Step-by-step explanation:
Sam spent 1/2 hour more on his history assignment compared to his English assignment. We can find this by subtracting the time he took on his English assignment (1/4 hour) from the time he spent on his history assignment (3/4 hour). The calculation is as follows:
3/4 hour - 1/4 hour = 2/4 hour, which simplifies to 1/2 hour
This means that Sam spent an extra half hour on his history assignment. Therefore, the correct answer is a. 1/2 hour.