Just try each word in the blank, and choose the one that makes the most sense.
Pablo was riding his bike to school when his strap broke. ✔
Pablo was ride his bike to school when his strap broke. ❌
Pablo was rodes his bike to school when his strap broke. ❌
Pablo was rides his bike to school when his strap broke. ❌
Answer is: Riding.
Pablo was riding his bike to school when his strap broke.
