If (a + b) : (a - b) = 13 : 3, a : b is :

1. $\dfrac{13}{3}$

2. $\dfrac{3}{13}$

3. $\dfrac{5}{8}$

4. $\dfrac{8}{5}$

Given,

$\Rightarrow \dfrac{a + b}{a - b} = \dfrac{13}{3} \\[1em] \Rightarrow 3(a + b) = 13(a - b) \\[1em] \Rightarrow 3a + 3b = 13a - 13b \\[1em] \Rightarrow 13a - 3a = 3b + 13b \\[1em] \Rightarrow 10a = 16b \\[1em] \Rightarrow \dfrac{a}{b} = \dfrac{16}{10} \\[1em] \Rightarrow \dfrac{a}{b} = \dfrac{8}{5}.$

Hence, Option 4 is the correct option.