Determine the gradient G of the graph and the spring constant k to a suitable number of significant figures, including units.

$$G = \frac{5.0 \text{ N} - 0.0 \text{ N}}{21.6 \text{ cm} - 0.0 \text{ cm}}$$ $$G = \frac{5.0 \text{ N}}{21.6 \text{ cm}}$$ $$G \approx 0.23148 \text{ N/cm}$$

The load values are given to 3 significant figures (e.g., 5.00 N), and the calculated extension values have 2 or 3 significant figures (e.g., 3.9 cm, 21.6 cm). Limiting to the least number of significant figures, we round to 2 significant figures.

$$G \approx 0.23 \text{ N/cm}$$

The gradient G of the graph is $\boxed{\text{0.23 N/cm}}$.

As determined in part (d), 2 significant figures are appropriate.

The spring constant $k$ is $\boxed{\text{0.23 N/cm}}$.