Final answer:
The four most viable options to make tacit collusion more likely are direct negotiation, price leadership, precommitments, and preannouncements.
Step-by-step explanation:
The four most viable options to make tacit collusion more likely are:
1. Direct negotiation: This involves direct communication between the firms to set price and output levels.
2. Price leadership: One firm takes the lead in setting prices and other firms follow suit.
3. Precommitments: Firms make binding commitments to limit output or maintain prices at a certain level.
4. Preannouncements: Firms publicly announce their intentions to maintain prices or limit output, encouraging other firms to follow suit.