To find out how many rosemary plants were planted, we begin by noting that the group planted four types of herbs: basil, thyme, rosemary, and oregano. Since they planted the same number of each type of herb and there are four types, we can express the total number of plants as follows:
Let \( x \) be the number of each type of herb planted. Since there are 4 types of herbs and the total is 52 plants, we can write the equation:
\[ 4x = 52 \]
Now, we solve for \( x \):
\[ x = \frac{52}{4} = 13 \]
Thus, each type of herb, including rosemary, consists of 13 plants. Therefore, the number of rosemary plants planted is:
\[ \text{Number of rosemary plants} = 13 \]
The correct response is:
13 rosemary plants.