Final answer:
In a spreadsheet, the structure that extends down a page vertically is called a column, which is labeled with letters and used to hold and analyze data.
Step-by-step explanation:
The part of a spreadsheet's structure that extends down a page vertically is called a column. In spreadsheets, columns are typically labeled with letters, starting with 'A' for the first column and continuing sequentially through the alphabet. Each column holds data that can be manipulated and analyzed within the spreadsheet application. For example, if you input numbers into a column, you can perform calculations on them, such as finding the sum or average.
In the context provided in the multiple choice question, the correct answer to "What is the part of a spreadsheet's structure that extends down a page vertically called?" is B. Column.