Final answer:
The approximate possible number of models when using the subset approach is 6⁶.
Step-by-step explanation:
The approximate possible number of models when using the subset approach can be calculated by taking the number of variables to the power of the number of elements in the subset. In this case, the number of variables, p, is 6. Therefore, the approximate possible number of models is 6 to the power of 6, which is equal to 46656.
Answer: The approximate possible number of models when using the subset approach is 6⁶ (option d).