In a town of 840 persons, 450 persons read Hindi, 300 read English and 200 both.

Total persons in a town is 840

Let H denote set of persons who read Hindi

Let E denote set of persons who read English

Then n(⋃) = 840, n(H) = 450, n(E) = 300

Then n(H⋃E)' = n(⋃) – n(H⋃E)

= n(⋃) – [n(H) + n(E) – n(H⋂E)]

= 840 – [450 + 300 – 200]

= 840 – 550

n(H⋃E)' = 290

Therefore, the number of persons who read neither is 290.

Hence, the correct answer is option B.