Which of the following is an example of unstructured data?

Unstructured data refers to information that does not have a predefined data model or is not organized in a predefined manner, making it difficult to analyze using traditional data processing tools. Text documents, such as reports or letters, and social media posts are prime examples of unstructured data because they consist of free-form text and often contain rich contextual information that is not easily categorized or indexed. This type of data lacks the structure of rows and columns that is typical of structured data formats.

In contrast, the other options represent structured or semi-structured data formats. Excel spreadsheets, while they can house unstructured elements (like comments or notes), primarily contain organized data in a tabular format that allows for easy manipulation and analysis. SQL databases are designed for storing structured data and allow complex queries against that structured information. Similarly, CSV files, which are also formatted in a tabular manner, categorize data into rows and columns, making them structured.

Thus, the key differentiation lies in the lack of a specific structure in text documents and social media posts, which confirms that these are indeed examples of unstructured data.