In the given diagram ΔABC ∼ ΔEFG. If ∠ABC = ∠EFG = 60°, then the length of the side FG is:

Given, ΔABC ∼ ΔEFG

So, the corresponding sides are in proportion.

$$\begin{aligned}\Rightarrow \frac{AB}{EF} = \frac{BC}{FG}\\[1em] \Rightarrow \frac{15}{75} = \frac{3}{FG}\\[1em] \Rightarrow \frac{1}{5} = \frac{3}{FG}\\[1em] \Rightarrow FG = 3 \times 5\\[1em] \Rightarrow FG = 15\text{ cm}\end{aligned}$$

Hence, option 1 is the correct option.