Google Inc.’s Android software has become the most popular operating system in the U.S. among new smartphone buyers, passing the iPhone and BlackBerry platforms, according to Nielsen Co.

Android was the top choice for U.S. consumers who bought a smartphone in the past six months with Research In Motion Ltd.’s BlackBerry and Apple Inc.’s iPhone tied for second place, Nielsen said.

Android has surged in popularity because the software is free to any company that wants to build phones using it and a wide range of manufacturers have adopted it, including Motorola Inc. and HTC Corp. Verizon Wireless, the largest U.S. wireless operator, has promoted Android devices, and AT&T Inc., the exclusive U.S. carrier for the iPhone, now also offers several Android models.

“Distribution and choice is king,” said Roger Entner, Nielsen’s head of telecom research. “They expand their distribution to Verizon, Sprint and AT&T and introduce a whole slew of devices and their market share goes up like a rocket.”

AT&T, the second-largest U.S. wireless operator, said today it will start selling three new Motorola Android phones, ranging in price from $79.99 to $129.99 with a two-year contract.

In August, Nielsen said that Android had passed Apple’s iPhone in sales among new U.S. smartphone buyers in the second quarter, though it still trailed the BlackBerry.

While BlackBerry retained the top spot among all U.S. smartphone owners with a 31 percent share, its lead over Apple is declining, Nielsen said. IPhone accounted for 28 percent of users and Android, 19 percent.

Google, based in Mountain View, California, climbed 1.9 percent to $532 at 10:27 a.m. in Nasdaq Stock Market trading. Google had dropped 16 percent this year before today, while RIM declined 28 percent and Apple jumped 32 percent.