A model building is built at a scale of 1:50. If the model is 8 centimeters tall, how tall is the actual building, in meters?

A $1:50$ scale means every length on the building is $50$ times its length on the model. The model is $8$ cm tall, so the building is $8\cdot50=400$ cm tall. Convert to meters by dividing by $100$: $400\div100=4$ meters, choice B. Choice D ($400$) leaves the answer in centimeters, though the question asks for meters. Choice C ($40$) divides by $10$ instead of $100$. Choice A ($0.4$) divides by $1000$. A four-meter building is short but reasonable for a model of a small structure, and the answer must be in the unit requested.