A machine makes large boxes or small boxes, one at a time, for a total of

Choice B is correct. It’s given that it takes the machine $10$ minutes to make a large box. It's also given that $x$ represents the possible number of large boxes the machine can make each day. Multiplying $10$ by $x$ gives $10x$, which represents the amount of time spent making large boxes. It’s given that it takes the machine $5$ minutes to make a small box. It's also given that $y$ represents the possible number of small boxes the machine can make each day. Multiplying $5$ by $y$ gives $5y$, which represents the amount of time spent making small boxes. Combining the amount of time spent making $x$ large boxes and $y$ small boxes yields $10x+5y$. It’s given that the machine makes boxes for a total of $700$ minutes each day. Therefore $10x+5y=700$ represents the possible number of large boxes, $x$, and small boxes, $y$, the machine can make each day.

Choice A is incorrect and may result from associating the time of $10$ minutes with small, rather than large, boxes and the time of $5$ minutes with large, rather than small, boxes.

Choice C is incorrect and may result from conceptual errors.

Choice D is incorrect and may result from conceptual errors.