What is a common reason the summary option may not be available in Outlook?

The selected answer is correct because the summary option in Outlook typically relies on having enough content within the message to create a meaningful and coherent summary. If the message is too short or lacks substantial information, the application may determine that it cannot generate a summary that would be useful or informative. Therefore, the absence of the summary option can be directly attributed to the insufficient content in that particular message.

In contrast, while a system error could theoretically lead to unavailability of certain features, it would not be a common or expected reason. Similarly, a long message wouldn't typically inhibit the ability to summarize; instead, it may even enhance it. As for an invalid recipient's email address, this would not have an impact on the summarization features within Outlook itself.