Correct Answer: $\sqrt[4]{5}$

Solution : Given: $\sqrt[6]{12},\sqrt[3]{4},\sqrt[4]{5},\sqrt3$
LCM of 6, 3, 4, 2 is 12.
$\sqrt[6]{12} ⇒ \left (12^\frac{1}{6} \right )^{12} = 12^2 = 144$
$\sqrt[3]{4} ⇒ \left (4^\frac{1}{3} \right )^{12} = 4^4 = 256$
$\sqrt[4]{5} ⇒ \left (5^\frac{1}{4} \right )^{12} = 5^3 = 125$
$\sqrt{3} ⇒ \left (3^\frac{1}{2} \right )^{12} = 3^6 = 729$
125 is the smallest of these numbers.
Hence, the correct answer is $\sqrt[4]{5}$.