Final answer:
The best verb tense to complete the sentence is the simple past tense verb 'worked' because the action is completed in the past.
Step-by-step explanation:
The best verb tense to complete the sentence 'On the last completed project, they '>worked for a month.' is the simple past tense verb 'worked'. This is because the action of working on the project is completed in the past. The present perfect tense 'have worked' would be used if the action was ongoing or had just been completed, and the future tense 'will work' and 'will have worked' are not appropriate because they refer to actions that will happen in the future or at a future point in the past, respectively.
Learn more about Verb tenses in English