Which of these triples can be the direction cosines of a line?

Explanation

1. 01

   Direction cosines satisfy $l^{2} + m^{2} + n^{2} = 1$.

2. 02

   $\frac{1}{2} + \frac{1}{4} + \frac{1}{4} = 1$; the others give $\frac{3}{4}$, 3 and $\frac{1}{3}$.