Yahoo! (YHOO): No chance. Yelp's market cap is now $3.5 billion, meaning a buyer would have to spend considerably more than that to buy it out. Yahoo! has about $6 billion in net cash and most of that is promised to shareholders through stock buybacks. Yahoo! can't afford Yelp.
Google (GOOGL): Little chance. There's enormous bad blood between Yelp and Google dating back to when Google tried to scrape all of Yelp's data from its site. Even though Google is nowhere in mobile, the history will be too tough to overcome.
Apple (AAPL): Longshot. This would be a big acquisition for Apple in terms of size. Apple has shown a preference for taking a "build it myself" approach to Apple Maps (where this would likely fit). Still, there has always been a close relationship between Apple and Yelp. Yelp CEO Jeremy Stoppelman has attended many Apple keynotes over the years.