The radical monobromination of bicyclo[2.2.1] heptane could give $\ce{1}$ ,$\ce{2}$ and $\ce{3}$.
However , $\ce{2}$ and $\ce{3}$ , are unstable .On observing $\ce{2}$ , planar radical at bridgehead is unstable due to angular strain introduced by planarity of radical at bridgehead .
Similarly in $\ce{3}$ , due to angular strain introduced in to the neighboring bonds,this radical also is unstable.
The situation leads to product from ${1}$ , that gives stereoisomers as shown below.
