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Choice C is correct. It's given that $f\left(x\right)$ is the total cost, in dollars, to lease a car from this dealership with a monthly payment of $x$ dollars. Therefore, the total cost, in dollars, to lease the car when the monthly payment is $\$400$ is represented by the value of $f\left(x\right)$ when $x=400$. Substituting $400$ for $x$ in the equation $f\left(x\right)=36x+\mathrm{1,000}$ yields $f\left(400\right)=36\left(400\right)+\mathrm{1,000}$, or $f\left(400\right)=\mathrm{15,400}$. Thus, when the monthly payment is $\$400$, the total cost to lease a car is $\$\mathrm{15,400}$. 

Choice A is incorrect and may result from conceptual or calculation errors.

Choice B is incorrect and may result from conceptual or calculation errors.

Choice D is incorrect and may result from conceptual or calculation errors.