Final answer:
A linear constraint is an inequality or equation that represents a straight line in a graph. Options (a), (c), and (d) are all examples of linear constraints because they can be represented by straight lines when graphed. Option (b) is incorrect.
Step-by-step explanation:
A linear constraint is an inequality or equation that represents a straight line in a graph. It restricts the solution space of a system of linear equations. In this case, the options (a), (c), and (d) are all examples of linear constraints because they can be represented by straight lines when graphed. To solve this, we can set up an equation to represent each person's charging structure and then equate them.
Option (b) is incorrect because it states that none of the others are linear constraints, which is not true.