# The basic radio antenna is a metal rod,the length of which is

$\begin{array}{1 1}(a)\;\lambda\; m\text{ at frequency of operation}\\(b)\;\large\frac{\lambda}{2}\normalsize m \text{ at frequency of operation}\\(c)\;\large\frac{\lambda}{4}\normalsize m \text{ at frequency of operation}\\(d)\;2\lambda\; m\text{ at frequency of operation}\end{array}$

Length of metal rod of antenna is $\large\frac{\lambda}{2}$m at the frequency of operation.
Hence (b) is the correct option.