Answer: C
Correct answer: the small square at the corner of a selection used to copy or extend a series
The Fill Handle is the small square at the bottom right corner of a selection. Dragging it copies the contents into adjacent cells, or extends a recognised series. It recognises a great many patterns. Dragging a single date advances it by a day; dragging Jan produces the months; dragging Monday produces the days. Entering two values first - 5 and 10 - establishes the step, so the fill continues 15, 20 and so on. Double-clicking the handle fills down as far as the adjacent column has data, which is much faster than dragging on a long list. Formulas filled this way have their relative references adjusted, which is exactly the behaviour absolute references with dollar signs exist to prevent. The Auto Fill Options button that appears afterwards offers to copy without formatting, fill formatting only, or fill as a series - useful when the default guess is wrong.