S1. The final outcome can never be predicted incontrovertibly. S2. All outcomes are based on variables that cannot always be fully understood. The second statement:

The first statement asserts that final outcomes cannot be predicted with certainty. The second statement provides the reason: outcomes depend on variables that cannot always be fully understood. The second statement directly explains and supports the claim in the first, providing a strong reason for the unpredictability mentioned. This is a case of full reinforcement — the second gives the underlying cause for the assertion in the first. 'Reinforces to an extent' is too weak; 'definitely follows' refers to logical entailment rather than support. Hence (a) fully reinforces the first is the best answer.