A model of torpedo is tested in a towing tank at a velocity of 25 m/sec. The prototype is expected to attain a velocity of 5 m/sec. What model scale has been used ? A. 1 : 5 B. 1 : 2.5 C. 1 :25 D. 1:50

The correct answer is $\boxed{\text{A. }1:5}$.

The model scale is the ratio of the prototype’s dimensions to the model’s dimensions. In this case, the prototype is expected to attain a velocity of 5 m/sec, while the model is tested at a velocity of 25 m/sec. This means that the model is 5 times smaller than the prototype. Therefore, the model scale is 1:5.

Option B is incorrect because the model is 5 times smaller than the prototype, not 2.5 times smaller.

Option C is incorrect because the model is 5 times smaller than the prototype, not 25 times smaller.

Option D is incorrect because the model is 5 times smaller than the prototype, not 50 times smaller.