Explanation:
<update> This tag does not belong to <URL> tag of the site map. Hence this option goes invalid.
<lastmod> This gives details of the date and time when the website content was changed. It will be in "W3C Date time format". This may omit time at times. This is the write answer.
<loc> This contains the URL of the web page.
<changefreq>: How frequently the "web page is likely to get modified". The values which it accepts are: "monthly, always , daily , weekly ,hourly, yearly , never"
