The correct answer to this question is C, 30. This type of question would be found on a test for early learners, such as kindergartners, who are just starting to learn math. It can test their comprehension and ability to solve problems. There are two ways that one could solve this question.
The most common way would know their time's tables. 5's are the easiest to learn, and if they know the order, 5, 10, 15, 20, 25, 30, they could solve. If not at the point to know their time's tables, one could solve it by identifying the sequence. Each number that increases is increased by 5. Adding 5 to 25 would get 30, which is the answer.