To process a number typed in a TextBox, the programmer must convert the Text value to a numeric data type. This is necessary because the Text property of a TextBox control returns a string value, which cannot be used in mathematical operations without first being converted.

Now let's go through each option and explain why it is right or wrong:

A. use the Val function to convert the Text value: This option is partially correct. The Val function is one way to convert a string to a numeric data type, but it is not the only way. The Val function converts a string to a Double data type, so it may not be appropriate if the value in the TextBox is an Integer.

B. use the CDbl function to convert the Text value: This option is partially correct. The CDbl function is another way to convert a string to a numeric data type, but it is not the only way. The CDbl function converts a string to a Double data type, so it may not be appropriate if the value in the TextBox is an Integer.

C. use the IsNumeric function to convert the Text value: This option is incorrect. The IsNumeric function does not convert a string to a numeric data type. Instead, it checks whether a value is numeric and returns a Boolean value.

D. Both a and b: This option is correct. Using either the Val or CDbl function can convert a string to a numeric data type. However, the programmer should choose the appropriate function depending on the data type of the value in the TextBox.

E. All of the above: This option is incorrect. While options A and B are correct, option C is not a valid way to convert a string to a numeric data type.

Therefore, the answer is: D. Both a and b.