Answer:
C) Kelly must sell 300 pairs of shoes each month.
Explanation:
Here, x represents the number of days she has worked that month,
Also, the number of of pairs of shoes that have left to sell after x months is,
![S=300-15x](https://img.qammunity.org/2020/formulas/mathematics/high-school/1xjnssto6vt0cgc7zcyh3xmjbgzwwcenr5.png)
Since, at the starting of month, x = 0,
So, the number of pair of shows she has to sell for a month,
![S=300-15* 0 = 300 - 0 = 300](https://img.qammunity.org/2020/formulas/mathematics/high-school/el1tuifr443r4jrpwl6og2n349smlq16l7.png)
Hence, 300 represents the pairs of shoes she must sell each month.
⇒ Option C is correct.