The cost of bearish Baidu Inc options fell to a five-year low on speculation China's largest Internet search engine will boost mobile advertising revenue and halt a three-month slump in its shares.

Puts protecting against a 10 percent decline in Baidu on Friday cost 1.4 points more than calls betting on a 10 percent gain, according to data on three-month options compiled by Bloomberg. The price relationship, known as skew, fell to 0.81 on April 24, the lowest since April 2008. US-traded shares of Baidu slumped 21 percent from the end of January through April on concern the company won't be able to maintain its leadership in the desktop Internet search market.

Baidu is extending its services to mobile devices as profit growth slumps to the slowest pace since its shares were listed in 2005. The company said this month it's buying PPS Net TV's Internet video business and will combine it with iQiyi.com, which it bought last year, to create China's largest online video website for mobile users and compete with dominant player Youku Tudou Inc.