Correct Answer - Option 3 : 90°

**Given:**

AB, BC and AC are the three sides of ΔABC of length 6√3 cm, 6 cm and 12 cm respectively

**Concept used:**

If in a ΔABC, AB^{2} + BC^{2} = AC^{2}

Then ΔABC is a rightangle triangle with ∠B = 90°

**Calculation:**

According to the concept, we have

AB2 + BC2 = {(6√3)^{2} + 6^{2}} cm^{2}

⇒ (108 + 36) cm^{2}

⇒ 144 cm2 ----(1)

And, AC^{2} = 12^{2 }cm2

⇒ 144 cm2 ----(2)

Here, equation (1) = equation(2)

So, AB2 + BC2 = AC^{2}

**∴ ∠B is 90°.**