The correct answer is:
**d. Parallel lines have the same slope.**
### Explanation:
- **a. Parallel lines have slopes that are undefined**: This is not true for all parallel lines. It is only true for vertical lines, which are a specific case. However, not all parallel lines are vertical.
- **b. Parallel lines have a slope of zero**: This statement applies to horizontal lines specifically, but not all parallel lines are horizontal. Therefore, this is also not true for all parallel lines.
- **c. Parallel lines have slopes that are negative reciprocals of each other**: This is false. Negative reciprocals apply to perpendicular lines, not parallel lines. For example, if one line has a slope of 2, a line perpendicular to it would have a slope of -1/2, while a parallel line would have the same slope of 2.
- **d. Parallel lines have the same slope**: This statement is true. By definition, parallel lines never intersect and therefore have identical slopes.
Thus, option **d** accurately describes the relationship between the slopes of parallel lines.