Right answer:
b. hablaría
This sentence is a Spanish conditional. The word si in the beginning of the sentence tells us that this is a conditional sentence. We use the conditional tense in Spanish when talking about hypothetical situations and probabilities or when making polite requests. So hablaría is the conjugation of the verb hablar that matches the third person singular in the conditional. Finally:
Si todos hablamos en español, Manolo hablaría inglés.