To answer this question, we need to understand the ratio of the question, which in this case is 3:4. If this ratio remains constant, then a simple way to solve this would be to multiply the 4 (in this case) so that it becomes 24 (because we know that these parts of the ratio correspond to each other). Since we know that 4*6=24, we can multiply both numbers included in the ratio by 6 to find our answer. After doing this, we find the ratio to be 18/24, which means that for every 24 buckets of gravel used, 18 buckets of sand are used.
So, the answer to this question is A. 18 buckets.
