Final answer:
Without the specific arrangement of the resistors in the question, it is not possible to determine the equivalent resistance. If the resistors were in parallel, the equivalent resistance would be less than the smallest individual resistance. A combination circuit requires a step-by-step analysis to find the equivalent resistance.
Step-by-step explanation:
To find the equivalent resistance between points a and b for resistors in combination, we need to know the specific arrangement of these resistors, which isn't provided in the question. However, assuming the question refers to the discussion in the given practice problems, the concept involves either series or parallel connections, or a combination of both.
For resistors in series, the equivalent resistance (Req) is the sum of the resistances. For resistors in parallel, the equivalent resistance can be calculated using the reciprocal formula: 1/Req = 1/R1 + 1/R2 + ... + 1/Rn. Complex circuits may require a step-by-step approach, reducing series and parallel parts progressively until one equivalent resistance is found.
If the question references the combination shown in 'Figure 19.19' of the practice problems where resistors are in parallel, the equivalent resistance would be less than the smallest individual resistance, as parallel circuits divide the total current and thus reduce the overall resistance. This is not one of the options provided. Since we don't have the figure, we cannot provide a definite answer to the original question.