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The correct answer is option 3, Martina Hingis. Leander Paes won the US Open Mixed Doubles Tennis Title in 2015 by partnering with Martina Hingis.
Option 1, Kristina Mladenovic, is incorrect because Leander Paes did not partner with her to win the US Open Mixed Doubles Tennis Title in 2015.
Option 2, Flavia Pennetta, is incorrect because Leander Paes did not partner with her to win the US Open Mixed Doubles Tennis Title in 2015.
Option 4, Sania Mirza, is also incorrect. Although Sania Mirza is an accomplished tennis player and has partnered with Leander Paes in the past, they did not win the US Open Mixed Doubles Title together in 2015.
Therefore, the correct answer is option 3, Martina Hingis. Leander Paes and Martina Hingis formed a successful partnership and won the US Open Mixed Doubles Tennis Title in 2015.