(C) Only I, II and III are correct.
Buddhism spread through cultural exchange, silk routes, and trade & travellers. European efforts had no role in its spread; this option is incorrect.
The textbook (Chapter 3, Section 1.1) states: "Buddhism emerged from eastern India and spread in several directions through intersecting points on the silk routes." The silk routes facilitated trade, travel, and cultural exchange simultaneously — so reasons I, II, and III are valid. Reason IV (European efforts) is not mentioned anywhere in the context of Buddhism's spread and is historically inaccurate for this period.