Final answer:
To find an equivalent ratio to 12/21, divide both terms by their greatest common divisor, which is 3, to get the simplified equivalent ratio of 4:7.
Step-by-step explanation:
To find an equivalent ratio for 12/21, we need to find a ratio that represents the same relationship between two numbers. One way to do this is by simplifying the ratio by finding a common divisor for both numbers. In this case, both 12 and 21 are divisible by 3.
Therefore, if we divide both terms of the ratio by 3, we get:
So, the equivalent ratio of 12/21 when simplified is 4:7, which is option (a).