Answer: "Whom" is the word that fills in the blank: "The award for excellence in architectural design will go to the architech WHOM the committee selects". The antecedent is "architect".
Step-by-step explanation: "Whom" is a relative pronoun used to introduce an adjective clause, that is to say a dependent clause which is included in a sentence to give more information about the antecedent. Moreover, "whom" refers to an object of the verb or the preposition. In the sentence provided, "whom" is the word that fills in the blank because it is acting as the direct object of the clause ("whom" can be replaced with "him" or "her"). Furthermore, "whom" is making reference to "architect", which is the antecedent.