Physics: Two boxes and a Cord?

The masses can be considered as a single mass, since they are connected. (The length of the cord can be arbitrarily short -- so short that the masses are effectively one.)

So the mass being accelerated is M₁ + M₂. F = Ma, so a = F/M = F/(M₁ + M₂).

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Both blocks have the same acceleration = F/(M1+M2) which is choice C. if you add the parenthesis. 

By Newton's Second Law, F = (M₁ + M₂)*a

and so the acceleration is a = F/(M₁ + M₂) ◄ C closest

option C written this way : a = F/(m1+m2) ...(parentheses do matter)