The correct answer is: A. each variable in one column.
Tidying data is a process of organizing data in a way that makes it easy to understand and analyze. The goal of tidying data is to create a data set that has one variable per column and one observation per row. This makes it easy to see the relationships between the variables and to identify patterns in the data.
Option B is incorrect because it does not specify that each variable must be in its own column. This could lead to confusion, as it would be difficult to tell which variables are related to each other.
Option C is incorrect because it does not specify that each observation must be in its own row. This could lead to confusion, as it would be difficult to tell which observations are related to each other.
Option D is incorrect because it does not specify any of the traits of tidy data.