Answer: C
Correct answer: a file that can contain several worksheets
A workbook is the file; a worksheet is one sheet of cells within it. A new workbook typically opens with one sheet, and more can be added, renamed, coloured, reordered, copied and hidden using the tabs at the bottom. The distinction matters in formulas, because a reference can cross sheets. =Sheet2!A1 reads a cell on another sheet of the same workbook, and =[Budget.xlsx]Sheet1!A1 reads one in a different workbook - a link that must be updated when the source changes. A 3-D reference spans several sheets at once: =SUM(Jan:Dec!B5) adds cell B5 across twelve monthly sheets, which is the standard way to consolidate identical sheets. Several sheets can be selected together with Ctrl or Shift, after which typing or formatting applies to all of them at once - useful, but the cause of a good deal of accidental damage when the grouping is forgotten.