Given that, ratio of seats $M:P:B = 5:7:8$
Let say $M = 50$, increased by $40\% = 50 + 50 \times \frac{40}{100} = 50 + 20 = 70$
$P = 70$, increased by $50\% = 70 + 70 \times \frac{50}{100} = 70 + 35 = 105$
$B = 80$, increased by $75\% = 80 + 80 \times \frac{75}{100} = 80 + 60 = 140$
$\therefore$ The required ratio $M:P;B = 70:105:140 = 2:3:4.$
So, the correct answer is $(A).$