B A is real-time monitoring of business changes in a short cycle, which belongs to daily briefing reports; C and D are summaries targeting a specific topic, belonging to special review reports. The correct answer is B.

正确答案是B: A company's semi-annual financial analysis conducted in July 2020。

专业分析如下：

A: Packet loss during upload and download on a website on January 1, 2020
- 这个选项描述的是一个具体时间点（2020年1月1日）的技术问题报告，不属于周期性回顾报告。

B: A company's semi-annual financial analysis conducted in July 2020
- 这个选项描述的是一个公司的半年度财务分析，这是对过去半年的业务行为和结果的回顾和总结，符合周期性回顾报告的定义。

C: An e-commerce platform's assessment report on the effectiveness of "Double 11" promotional campaign
- 这个选项描述的是对某个特定促销活动（"双十一"）效果的评估报告，虽然也是对过去行为的回顾，但它是针对特定事件而非一个周期。

D: An e-commerce platform's analysis report on payment behavior characteristics of different channel user groups during "618" event
- 这个选项描述的是对特定事件（"618"）期间用户支付行为特征的分析报告，同样是针对特定事件而非一个周期。

因此，B选项是唯一一个符合周期性回顾报告定义的选项。