To answer this question, the user needs to understand the behavior of group functions when used on columns that contain NULL values.

Option A is correct. Group functions ignore NULL values in columns. For example, the AVG function calculates the average of all non-NULL values, while the COUNT function counts all non-NULL values. This means that NULL values are not included in the calculation and do not affect the result.

Option B and Option C are incorrect because they describe the behavior of specific group functions, rather than the behavior of group functions in general. The behavior of each group function depends on the specific function being used and the data type of the column being aggregated.

Option D is incorrect because group functions can be accurately used on columns that contain NULL values. However, it is important to keep in mind that NULL values may affect the result of certain operations, such as the SUM function. If a column contains both NULL and non-NULL values, the SUM function will return a NULL value, unless the column is aggregated using the NVL function or a similar function that replaces NULL values with a default value.

Therefore, the correct answer is:

The Answer is: A. Group functions on columns ignore NULL values.