To trigger the validate() method of an ActionForm, you need to set the validate attribute to "true".
Explanation of each option:
A. validate="false": This option will not trigger the validate() method of the ActionForm. It disables the validation process.
B. validate="true": This option will trigger the validate() method of the ActionForm. It enables the validation process.
C. validate="yes": This option is not a valid attribute value for the validate attribute. It will not trigger the validate() method.
D. validate="no": This option is not a valid attribute value for the validate attribute. It will not trigger the validate() method.
Therefore, the correct answer is B) validate="true". This option enables the validation process and triggers the validate() method of the ActionForm.