How should "etc." be used correctly in a sentence?

The choice indicating the use of "etc." in lists is correct because "etc." (short for the Latin phrase "et cetera") is used to signify that there are additional items in a list that are not explicitly mentioned. Its primary function is to avoid redundancy by indicating that similar items could follow without needing to list every single one. For example, when saying "the list included diseases such as measles, flu, chicken pox, etc.," the use of "etc." effectively communicates to the reader that there are other diseases that are related to those mentioned.

In formal writing, the use of "etc." is typically acceptable, but it is often encouraged to provide a complete list or use alternatives like "and so forth" or "and others" to maintain clarity. Therefore, while it can be found in formal writing, it is not limited explicitly to that context. Starting a sentence with "etc." is generally discouraged because it requires prior context for understanding, making this usage less effective. Lastly, while "etc." might be used in casual conversation, it is not reserved for that setting alone. Thus, the most appropriate and correct usage is within lists, as highlighted in the selected answer.