In a correlational study, . A. one variable is measured and two groups are compared
B. two variables are measured and two groups are compared
C. one variable is measured and there is only one group of participants
D. two variables are measured and there is only one group of participants

Option B,  two variables are measured and two groups are compared

Step-by-step explanation:

In a correlation study relationship between two variable is established.

The two variables represents two group.

For instance correlation study is conducted to determine the relationship between two variables X and Y

X and Y are variable

While X represents the group "number of cars passing through a lane"

Y represents the group "width of the road".

Hence, Option B is correct