Highlighting Search Keyword

you will need to have edit option to the search results web part for this requirement.

1. Go to Site Actions --> Edit Page
2. Click on edit --> Modify shared web part of the Core Search Results Web Part.
3. Click on the XSL editor to view the XSLT.
4. Look for this code
5. Place the bold tag between a span tag i.e.
6. Click on OK and OK on the we part properties.
7. Now you can see the keyword highlighted in yellow color.