To find the marks Raju must obtain in chemistry to achieve an average of 80 marks across three subjects (mathematics, physics, chemistry), we need to calculate the total marks he should have and then solve for the chemistry marks.

1. Raju's marks in mathematics are 75, and in physics, they are 85.
2. We are tasked to find the chemistry marks, denoted as x.
3. To have an average of 80 marks in all three subjects, the total marks for the three subjects should be:

\text{Average} = \frac{\text{Total Marks}}{\text{Number of Subjects}}

4. Plug in the known values:

80 = \frac{75 + 85 + x}{3}

5. To find the total required marks, multiply both sides by 3:

240 = 75 + 85 + x

6. Calculate the total of mathematics and physics marks:

75 + 85 = 160

7. Replace the sum and solve for x:

240 = 160 + x

8. Subtract 160 from both sides:

x = 240 - 160 = 80

9. Therefore, Raju must score 80 in chemistry to have an average of 80.

Thus, the correct answer is 80.