For a European call on a dividend-paying stock, the lower bound is

$$c \ge S_0 - D - Ke^{-rT}$$

where $D$ is the present value of dividends paid during the life of the option.

Here:

• $S_0 = 24$
• Dividend = `$1.00` paid in 4 months
• $r = 5\%$
• $T = 0.5$ years

Present value of dividend:

$$D = 1 \times e^{-0.05 \times \frac{4}{12}} \approx 0.9835$$

Then:

$$c \ge 24 - 0.9835 - 20e^{-0.05 \times 0.5}$$ $$= 24 - 0.9835 - 20e^{-0.025} \approx 24 - 0.9835 - 19.5062 = 3.5103$$

Therefore, the lower bound is approximately $3.51.