If l : m : n = 1 : 2 : 4, then √(5l^2 + m^2 + n^2) is equal to:

If $l : m : n = 1 : 2 : 4$,

then $\sqrt{5l^2 + m^2 + n^2}$ is equal to = ?

Put the values in required equation,

$\sqrt{5(1)^2 + (2)^2 + (4)^2}$

= $\sqrt{5 + 4 + 16}$  = 5

So ,

we know the value of l = 1

So, according to the options the answer will be 5l