b
According to $Wein's$ displacement law,
${\lambda _{\max }}T = constant$
$\therefore \frac{{{\lambda _{{{\max }_1}}}}}{{{\lambda _{{{\max }_2}}}}} = \frac{{{T_2}}}{{{T_1}}}$
$or,\,\,{\lambda _{{{\max }_2}}} = \frac{{{\lambda _{{{\max }_1}}} \times {T_1}}}{{{T_2}}} = \frac{{5000 \times 1500}}{{2500}} = 3000\,\mathop A\limits^ \circ .$