To solve this question, the user needs to have knowledge of the concept of percentages and how to solve for an unknown percentage value.

We can start by using the given information that 84% of a total is 630 marks. We can set up the following equation:

0.84x = 630

Where x is the total number of marks. We can solve for x by dividing both sides by 0.84:

x = 630 ÷ 0.84
x = 750

Therefore, the total number of marks is 750.

Next, we need to find what 90% of 750 is. We can set up the following equation:

0.9 × 750 = 675

Therefore, 90% of the total is equal to 675.

Now, let's go through each option and explain why it is right or wrong:

A. 500: This option is incorrect. 90% of 750 is greater than 500.

B. 550: This option is incorrect. 90% of 750 is greater than 550.

C. 675: This option is correct. We found that 90% of 750 is equal to 675.

D. 425: This option is incorrect. 90% of 750 is greater than 425.

Therefore, the answer is:

The Answer is: C