To find the marks Raju must get in chemistry to have an average of 80 marks across mathematics, physics, and chemistry, we can use the formula for calculating the average. The average of three subjects is given by:

\text{Average} = \frac{\text{Sum of scores in all subjects}}{\text{Number of subjects}}

We know:

• Marks in Mathematics = 75
• Marks in Physics = 85
• Required average = 80

Let the marks Raju needs in chemistry be x. Therefore, the equation for the average becomes:

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

Solving this equation:

75 + 85 + x = 80 \times 3

160 + x = 240

x = 240 - 160

x = 80

Therefore, Raju must score 80 in chemistry to have an average of 80 marks in all three subjects.

This matches the given correct answer, so option B, 80, is correct.

To ensure clarity:

• x = 80 ensures the sum of marks = 75 + 85 + 80 = 240, and dividing by 3 gives the average as 80.