The correct answer is option 3, Greeks. Yavanika, which means curtain, was introduced in Indian theatre by the Greeks. The introduction of the yavanika marked a significant change in Indian theatre as it allowed for the use of scene changes and reveals. The curtain was used to separate the stage from the audience and was operated by a system of ropes and pulleys. This innovation brought a sense of dramatic effect and suspense to the performances. The Greeks had a strong influence on Indian theatre, introducing various concepts and techniques, including the use of masks and amphitheatres. It is important to note that while the Greeks introduced the yavanika in Indian theatre, other ancient civilizations like the Shakas, Parthians, and Kushans had their own contributions to theatre and performance arts.