A European-style option can be exercised only at expiration under its contract rules. It can ordinarily be bought or sold before then while its market is open; exercise style does not require holding it to expiration.
Many index options and some futures options use European exercise, but the style is product-specific. Check settlement method, last trading time, observation price and exercise instructions for the exact series.
Content reviewed:
Primary references: OIC exercise guidance; CME futures options. Editorial standards.