The fourth proportional to 3, 4, 5 is

1. 6

2. $\dfrac{20}{3}$

3. $\dfrac{15}{4}$

4. $\dfrac{12}{5}$

Let the fourth proportional be x.

So, the numbers 3, 4, 5, x are in proportion.

$\therefore 3 : 4 : : 5 : x \\[0.5em] \Rightarrow \dfrac{3}{4} = \dfrac{5}{x} \\[0.5em] \Rightarrow x = 5 \times \dfrac{4}{3} \\[0.5em] \Rightarrow x = \dfrac{20}{3}.$

∴ Option 2 is the correct option.