The correct statement is:
A) All of the class is willing to take part in the prank.
Here's a brief explanation of why the other options are incorrect:
B) "All of the classes is willing to take part in the prank." - The subject "classes" is plural, so it should be "are" instead of "is."
C) "All of the class are willing to take part in the prank." - The subject "class" is singular, so it should be "is" instead of "are."
D) "All of the classes has been willing to take part in the prank." - The subject "classes" is plural, so it should be "have" instead of "has."