An activity-based costing system that is designed for internal decision-making will not conform to generally accepted accounting principles because:

The reasons why an activity-based costing system that is designed for internal decision-making will not uphold to the generally accepted principles of accounting are the following:
1. Some manufacturing costs like for example, the costs of idle capacity and organization-sustaining costs, will not be assigned to products.
2. some non-manufacturing costs are assigned to products.
3. first-stage allocations may be based on subjective interview data.

Therefore, the answer to this question is this: 
"all of the above are reasons why an activity-based costing system that is designed for internal decision-making will not conform to generally accepted accounting principles."