$v_b=v_{\large bird}=5 m/s;\quad v_T=v_{\large train}=-20 m/s$
Relative velocity of bird with respect to train
$=v_b-v_T$
$=5-(-20)$
$=25 m/s$
Time taken by bird to cross train = $\frac{150}{25} = 6s$
The bird take 6s to cross the train
Hence a is the correct answer.