** Shares of ad-tech firm Trade Desk TTD.O jump 13.9% to $85.9 premarket
** Stock on track to open at highest level in over five months, if gains hold
** TTD will replace engineering design firm Ansys ANSS.O in the S&P 500 .SPX, effective before markets open on Friday, July 18
** China's market regulator on Monday conditionally approved U.S. chip design software provider Synopsys' SNPS.O acquisition of ANSS
** Synopsys said on Monday that the companies now expect to close the transaction on or around July 17
** Stephens analyst Melissa Roberts estimates passive funds that track the S&P 500 will need to buy nearly 76 mln TTD shares, implying about nine days of buying demand
** As of Monday's close, TTD down more than 35% YTD, compared with a 8.7% rise in the Nasdaq 100 .NDX index
(Reporting by Shashwat Chauhan in Bengaluru)
((Shashwat.Chauhan@thomsonreuters.com))