The correct answer is D. MAX.

The MAX function in SQL can operate on any datatype. It is used to retrieve the maximum value from a column or a set of values. Regardless of the datatype of the column or the values, the MAX function will return the maximum value.

The other functions mentioned:

A. LOWER: It is used to convert a string to lowercase. It can only operate on string or text datatypes.

B. TO_CHAR: It is used to convert a value to a character string. It is typically used to convert a date or a number to a specific string format. It can operate on different datatypes but is primarily used for date and number conversions.

C. LPAD: It is used to pad a string with specified characters on the left side. It can only operate on string or text datatypes.

E. CEIL: It is used to round up a number to the nearest integer. It can only operate on numeric datatypes.

Therefore, out of all the options, only the MAX function can operate on any datatype.