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Explanation:

(1 + 2 + 3 + … + n) + x = 1200

Where, x is the roll number of the student who got twice as many as compared to his entitlement.

n (n + 1)/2 = 1200 – x ⇒ n (n + 1)

= 2400 – 2x
 
Substituting values of x from options, only for x = 24, the RHS can be expressed as a product of two natural numbers.

Hence, option (b).

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