Final answer:
In spreadsheet programs, the part that extends across the page horizontally is known as a row. Rows allow for data input and organization, and they run horizontally across the spreadsheet.
Step-by-step explanation:
The part of a spreadsheet's structure that extends across a page horizontally is called a row. In the context of spreadsheet programs such as Microsoft Excel or Goo-gle Sheets, a row is a horizontal series of cells. Users can input and organize data within these rows. For example, if you're working on a budget spreadsheet, you might use rows to represent different expenses, with each row holding information about a single type of expense.