Explanation:

- R can complete the work in 4 days, so R's rate is 1/4 of the work per day.

- S can complete the work in 6 days, so S's rate is 1/6 of the work per day.

- T can complete the work in 18 days, so T's rate is 1/18 of the work per day.

- When working together, their combined rate is: \( \frac{1}{4} + \frac{1}{6} + \frac{1}{18} \).

- This becomes \( \frac{9}{36} + \frac{6}{36} + \frac{2}{36} = \frac{17}{36} \) of the work per day.

- Thus, they can complete the work in \( \frac{36}{17} \approx 2.12 \) days when working together.

Option 2: 2.12 is the correct answer.