While a cell is in Edit Mode, pressing Enter normally confirms the entry and moves the cursor to the next cell, so a special key combination is needed to break the text onto a new line within the same cell.

Alt + Enter inserts a manual line break inside the current cell, letting long text or a multi-line formula wrap onto several lines while still occupying one cell.

This is the standard Excel shortcut for forcing a new line within a cell without ending the edit, which is precisely what the question asks.

Ctrl + Enter enters the same value into every cell in the current selection at once; it does not create a line break inside a single cell.

Shift + Enter confirms the entry and moves the active cell upward to the previous cell, so it also does not add a line break.

Tab + Enter is not a single line-break command: Tab moves the cursor one cell to the right and Enter confirms, so together they do nothing to break a line.

Because only Alt + Enter inserts an in-cell line break, the correct answer is Alt + Enter.