Final answer:
The simplest ratio of triangles to circles is found by reducing the ratio that can be simplified, which is option c) 9:15 (reducing to 3:5) corresponding to option a) as the simplest ratio of 3:5.
Step-by-step explanation:
The question asks for the simplest ratio of triangles to circles. To find this, we need to look for the option that can no longer be reduced by dividing both numbers by a common factor. Among the given options, option c) 9:15 is reducible, as both 9 and 15 can be divided by the common factor 3.
When we divide both numbers by 3, the ratio 9:15 simplifies to 3:5. Thus, option c) 9:15 can be reduced to 3:5, which corresponds to option a) as the simplest ratio. Since none of the other options can be reduced further, we conclude that the correct answer is option a), 3:5.