The correct answer is D. Riding
Explanation:
In sentences, the verb or word that completes an idea depends on the context provided. In the case of the sentence "Pablo was _____ his bike to school when his strap broke" the auxiliary verb "was" suggests the event occurred in the past and was a continuous action because only in the past continuous tense the verb includes the auxiliar "was/were" followed by an -ing verb as in "I was going to the cinema".
This means the verb that correctly completes the sentence needs to end in -ing and therefore the correct word is "riding". Additionally, this tense is often used to explain an action in the past that was occurring before another action occurred which is applies to the sentence because the strap fo the bike broke while Pablo was riding.
