a.) Erica los tiene en la bolsa.
b.) The direct object pronoun "los" is correct because "bloques" is a masculine plural noun. In Spanish, the direct object pronoun must agree in gender and number with the noun it replaces. "Los" is the plural form of the masculine direct object pronoun, which corresponds to "bloques" in this case. Thus, when substituting "bloques" in the original sentence, it is replaced by "los."