(Reuters) – Nielsen Holdings said on Sunday that the company has rejected an unsolicited acquisition proposal from a private equity consortium that valued the TV ratings company at $9.13 billion. The consortium had proposed to acquire Nielsen at $25.40 per share, which the board unanimously determined would significantly under value the company, it said in a statement. Earlier this month the Wall Street Journal reported that a consortium of private equity firms that included Elliott Management was is in talks to buy the company for about $15 billion including debt. Activist investor Elliott ha…