According to the instructions or guidelines provided in the text, particularly "・申し込み方法：10月1日までに、電話または直接センターの窓口でお申し込みください", the required action or next step is clearly defined. Therefore, the option "10月1日までに、電話か窓口で申し込む。" is the only correct choice as it matches the specified rules. A successful strategy for action-based questions is to carefully match dates, times, or conditional rules in the text to avoid distractors. The incorrect options violate these specific conditions or outline steps that are not recommended in the passage.