To answer this question, the user needs to have knowledge about the features supported by MSIE 3.x.

The correct answer is:

B. document.clear()

Option B is correct because the document.clear() method is supported in MSIE 3.x. This method clears the contents of the current document and is used to remove all the elements from the document.

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

A. split(): This method is used to split a string into an array of substrings. However, this method is not supported in MSIE 3.x.

B. document.clear(): This method is used to clear the contents of the current document. This method is supported in MSIE 3.x.

C. join(): This method is used to join all elements of an array into a string. However, this method is not supported in MSIE 3.x.

D. charAt(): This method is used to return the character at a specified index in a string. This method is supported in MSIE 3.x, but it is not the correct answer to this question.

Therefore, the correct answer is B.

The Answer is: B. document.clear()