Explanation:

The question describes a visual cue in Microsoft Excel: a cell with a bold boundary. This visual indicator signifies the currently selected cell that is ready for interaction. Option A, 'Relative', typically refers to relative cell references in formulas (e.g., A1), which adjust when copied to other cells. It does not describe the visual state of a selected cell. Option B, an 'Active' cell, is the cell that is currently selected in an Excel worksheet. It is distinguished by a thick, bold border, and its address is displayed in the Name Box. This is precisely what the question describes. Option C, 'Absolute', refers to absolute cell references in formulas (e.g., $A$1), which remain fixed when copied. It is unrelated to the visual selection of a cell. Option D, 'Mixed', refers to mixed cell references (e.g., A$1$ or $A$1), combining relative and absolute components. This also does not describe the visual state of a selected cell. Therefore, the cell with a bold boundary in Excel is known as the active cell.