Final answer:
To know the number of users active today and last month in Microsoft 365 Usage Analytics, use the Monthly Active Users (MAU) metric as it counts unique users active over the last 30 days.
Step-by-step explanation:
If you are using Microsoft 365 Usage Analytics, the metric you should use to determine the number of users who are active today and were also active last month is A) Monthly Active Users (MAU). The MAU metric provides the count of unique users who have performed an activity within the last 30 days. Although DAU would give you the number of daily active users, it wouldn't encompass last month's activity. Similarly, WAU and YAU would not provide the specific cross-section of data you're looking for, as WAU is measured over a week and YAU is measured annually. Thus, MAU is the appropriate metric as it will include users who have been active at any point during the last month, up until today.