Chapter 3 - Linear and Quadratic Functions - 3.1 Properties of Linear Functions and Linear Models - 3.1 Assess Your Understanding - Page 126: 12

$\text{(d)} \quad N(x)=40x+12,500$

Step $1$. The existing $12,500$ miles is the value at $x=0$, thus it is the $y$-intercept. Step $2$. The average rate of $40$ miles per day is the slope of the linear funtion. Step $3$. Thus, the function that models the odometer is $N(x)=40x+12,500$. The answer is option $\text{(d)}$.