Relationship Analysis:
The relationship between the words "promptu" and "rehearsal" is that of a type and its preparation. In other words, "promptu" refers to something that is done without preparation, and "rehearsal" refers to the act of preparing for a performance or event. Therefore, the relationship between the two words is that of a preparation and the action or event for which it is being prepared.

Candidate Pairs Analysis:
Let's analyze each of the candidate pairs of words to determine which one expresses a similar relationship to the original pair.

a) Numerical: Calculation
This pair does not express a similar relationship to the original pair. "Numerical" refers to numbers or numerical values, while "calculation" refers to the act of performing mathematical operations. The relationship between the two words is that of a subject and an action related to that subject, not a preparation and an action or event.

b) Affable: Permissiveness
This pair does not express a similar relationship to the original pair. "Affable" refers to someone who is friendly or easy to approach, while "permissiveness" refers to the act of allowing or granting permission. The relationship between the two words is that of a characteristic and an action or behavior associated with that characteristic, not a preparation and an action or event.

c) Menacing: Ambition
This pair expresses a similar relationship to the original pair. "Menacing" refers to something that is threatening or intimidating, while "ambition" refers to a strong desire or determination to achieve something. The relationship between the two words is that of a characteristic or state and a motivation or drive associated with that characteristic or state. Similarly, in the original pair, "promptu" refers to something that is done without preparation, and "rehearsal" refers to the act of preparing for a performance or event.

d) Digressive: Conciseness
This pair does not express a similar relationship to the original pair. "Digressive" refers to deviating from the main topic or going off on tangents, while "conciseness" refers to the quality of being brief and to the point. The relationship between the two words is that of an action or behavior and a quality or characteristic associated with that action or behavior, not a preparation and an action or event.

Conclusion:
The pair that best expresses a relationship similar to that expressed in the original pair is "Menacing: Ambition." This pair has a similar relationship of a characteristic or state and a motivation or drive associated with that characteristic or state.