To answer this question, we need to understand the features supported in MSIE 3.x.
Option A) split() - This option is incorrect because the split() function is not supported in MSIE 3.x.
Option B) document.clear() - This option is incorrect because the document.clear() function is not supported in MSIE 3.x.
Option C) join() - This option is incorrect because the join() function is not supported in MSIE 3.x.
Option D) charAt() - This option is correct because the charAt() function is supported in MSIE 3.x. The charAt() function is used to retrieve the character at a specified index in a string.
The correct answer is D) charAt() because it is the only feature supported in MSIE 3.x.