The correct answer is B. Create a Chatter stream.
A Chatter stream gives users a combined feed from selected people, groups, and records. Salesforce documentation states that users can create a Chatter stream to get a combined view of feeds from selected profiles, groups, and records.
For this requirement, the sales rep can create a stream that includes only the key Account and Opportunity records, so their Chatter feed is filtered to those important records.
Why others are incorrect:
A. Chatter bookmarks save individual posts, but they do not create a filtered feed.
C. Chatter notification alerts users, but it does not filter the feed.
D. Chatter group is for group collaboration, not filtering posts from selected records.
===========