A is the correct answer:
<apex:page standardController="Account">
<apex:pageBlock>
You're looking at some related lists for {!account.name}:
</apex:pageBlock>

<apex:relatedList list="Opportunities" />

<apex:relatedList list="Contacts">
<apex:facet name="header">Titles can be overriden with facets</apex:facet>
</apex:relatedList>

<apex:relatedList list="Cases" title="Or you can keep the image, but change the text" />
</apex:page>

B is incorrect:
Use controller extensions when: You want to leverage the built-in functionality of a standard controller but override one or more actions, such as edit, view, save, or delete.

B is correct answer