The most effective and standard method to achieve this reporting goal in Salesforce is by using a Campaign Hierarchy. A parent Campaign should be created for the overall series (e.g., "Q1 Networking Series"). Each monthly event would then be created as a separate child Campaign linked to this parent. This structure allows key metrics, such as costs, responses, and revenue from Opportunities, to automatically roll up from the child (monthly) campaigns to the parent (series) campaign. This directly facilitates simplified reporting on Campaign ROI for both individual events and the entire series.

A. Create individual Campaigns that all have the same name.

This approach creates data confusion and makes it difficult to distinguish between monthly events in reports. It provides no mechanism for aggregating data for the entire series.

B. Configure Campaign Member Statuses to record which event Members attended.

Campaign Member Statuses are designed to track a member's engagement within a single campaign (e.g., Sent, Responded), not to differentiate between multiple distinct events.

D. Add different record types for the monthly event types.

Record types control page layouts and picklist values; they do not create a relationship between records for the purpose of aggregate reporting or calculating a series-level ROI.

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1. Salesforce Help Documentation, "Campaign Hierarchies":

Reference: "Group campaigns into a hierarchy to organize your marketing initiatives. Then, you can analyze the marketing efforts in a specific region, of a certain type, or that are focused on a particular product. Statistics from the child campaigns roll up to the parent, so you can see aggregated data for the entire hierarchy." This directly supports using a parent/child structure for aggregate reporting.

2. Salesforce Help Documentation, "View Campaign Hierarchy Statistics":

Reference: This document details the specific fields that are aggregated from child campaigns to the parent, including Total Responses in Hierarchy, Total Value Won Opportunities in Hierarchy, and Total Cost in Hierarchy. These aggregated fields are essential for calculating the overall series ROI, confirming the validity of the hierarchy approach.

3. Trailhead by Salesforce, "Campaign Basics" Module, "Track Campaigns" Unit:

Reference: "To associate campaigns with one another, use the Parent Campaign field. Parent campaigns are great for grouping campaigns together. For example, if you have a series of webinars, you can create a campaign for each webinar and group them all under one parent campaign." This provides a clear, official example mirroring the user's scenario.