Explanation
Equivalence partitioning is a black-box test design technique that divides the input domain of a system into classes of data from which test cases can be derived. The idea is that if a system works correctly for a representative value from an equivalence class, it will work correctly for all values from that class, and vice versa. Equivalence partitioning reduces the number of test cases by eliminating redundant ones. For the given user story, equivalence partitioning is the most suitable technique because it can test the different outcomes of the annual car tax calculation based on the power of the vehicle, which is the main input for the system. By testing one value from each partition, the tester can verify the functionality of the system and detect any errors in the calculation logic. The other techniques are not as suitable because they do not focus on the inputdomain of the system, but rather on the conditions, transitions, or scenarios that are not directly related to the user story. References:
1: ISTQB Foundation Level Agile Tester Syllabus, Version 2014, Section 2.2.2
2: ASTQB Agile Tester Certification Resources, Agile Testing Foundations, Chapter 3, Section 3.2.2: 3