In the given figure,

AB = 10 cm, CD = 8 cm = OB, then OD is equal to :

1. 10 cm

2. 3.2 cm

3. 6.4 cm

4. 8 cm

From figure,

In △ AOB and △ COD,

⇒ ∠AOB = ∠COD (Vertically opposite angles are equal)

⇒ ∠OAB = ∠OCD (Alternate angles are equal)

∴ △ AOB ~ △ COD (By A.A. postulate)

We know that,

Corresponding sides of similar triangle are in proportion.

$\therefore \dfrac{OB}{OD} = \dfrac{AB}{CD} \\[1em] \Rightarrow \dfrac{8}{OD} = \dfrac{10}{8} \\[1em] \Rightarrow OD = \dfrac{8 \times 8}{10} \\[1em] \Rightarrow OD = \dfrac{64}{10} = 6.4 \text{ cm}$

Hence, Option 3 is the correct option.