$v_b=v_{\large bird}=5 m/s;\quad v_T=v_{\large train}=-20 m/s$

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.