A student has obtained 75%, 80%, and 85% in three subjects. If the marks of another subject is added then his average cannot be less than
$
  {\text{A}}{\text{. }}60\% \\
  {\text{B}}{\text{. }}65\% \\
  {\text{C}}{\text{. }}80\% \\
  {\text{D}}{\text{. }}90\% \\
 $

Hint: - Here we have to go through the properties of solving the question of average as we know average is the central or typical value in a set of data which is calculated by dividing the sum of the values in the set by their number.

Complete step-by-step answer:
Here in the question it is given that a student has obtained 75%, 80%, and 85% in three subjects.
If we see the percentage as a marks of that subject we can say that Marks obtained from 3 subjects out of 300 is 75+80+85=240
Now if we add another subject it means that the total marks of 4 subjects become 400.
And the marks of three subjects are given that is 240. It means the marks of the fourth subject lies from 0 to 100. But for the minimum the marks of the fourth subject must be zero.
i.e. marks ≥ 240 out of 400
Now if we calculate the average of fourth subject after adding the minimum marks of fourth subject it becomes,
$ \Rightarrow \dfrac{{240}}{4} = 60$(When marks are ${4^{th}}$ subject is 0).
Hence option A is the correct answer.

Note: - Whenever we face such a type of question the key concept for solving the question is to first let the percentage as marks out of 100 for easy solving then proceed it according to the question to find out the answer.