Answer: storesDF.withColumn("employeesPerSqft", col("numberOfEmployees") / col("sqft"))

The correct answer is A. The `withColumn` method is used to add a new column to a DataFrame, and it requires the name of the new column as a string and an expression that defines the column's values. In option A, the expression correctly uses the `col` function to reference the existing columns `numberOfEmployees` and `sqft`, and performs the division operation to create the new column `employeesPerSqft`. Option B is incorrect because it attempts to divide strings directly, which is not valid. Option C and D are incorrect because they use the `select` method, which would not preserve the existing columns of `storesDF` unless explicitly included in the `select` statement, and their syntax for creating the new column is not valid. Option E is incorrect because it passes a `Column` object as the first argument to `withColumn` instead of a string, which is not the expected method signature.

Author: LeetQuiz Editorial Team