Which of the following activities is an example of recreational mathematics?

Concept: Recreational mathematics refers to mathematical activity pursued primarily for enjoyment, curiosity, and intellectual challenge — puzzles, games, and brain-teasers explored voluntarily — rather than mathematics done as coursework, rote drilling, or exam-directed practice.

Application: Solving a Sudoku puzzle fits this definition directly. A Sudoku grid is filled by deducing, step by step, which digit can legally occupy each cell so that every row, column, and 3×3 box contains each digit exactly once. It is undertaken voluntarily as a logic puzzle, not because a syllabus requires it.

Contrast: The remaining activities are structurally different from a voluntary puzzle:

• Listening to a lecture without any interaction is one-way delivery of pre-structured content from a speaker; the listener is not working through a puzzle or game.

• Writing multiplication tables repeatedly is a repetition drill that reproduces a fixed, already-known sequence of products to build recall speed, not an open problem to be deduced.

• Copying formulas from a textbook is transcription of existing printed content without deriving, altering, or applying anything new.

Result: Among the four, only solving a Sudoku puzzle is an activity pursued as a puzzle for its own sake, so it is the example of recreational mathematics.