Answer:
Option A.
Explanation:
Given : ratio of red marbles to blue marbles in a bag is equal to 3 to 4 .
We need to find which of the options: A, B, C and D is true .
Here, 3 + 4 = 7
Therefore, sum of number of red marbles and number of blue marbles should be multiple of 7 ,hence the answer has to be a multiple of 7.
Now, option B is 30 which is not multiple of 7 .
Option C is 32 which is not multiple of 7 .
Option D is 40 which is not multiple of 7.
Option A is 21 which is a multiple of 7 .
Therefore, the only multiple of 7 on the list is A. 21
So, total number of marbles can be 21 .
Therefore the answer is A