• Number of Possible Unrooted Trees:
• For $n$ operational taxonomic units (taxa), the number of distinct unrooted bifurcating phylogenetic trees ($N_u$) is given by the double factorial formula:

$$N_u = (2n - 5)!! = 1 \times 3 \times 5 \times \dots \times (2n - 5)$$

• For $n = 5$ taxa:

$$N_u = (2 \times 5 - 5)!! = 5!! = 1 \times 3 \times 5 = 15$$

• (By comparison, the number of rooted trees for $n = 5$ is $(2n - 3)!! = 7!! = 105$).
• Conclusion: The number of unrooted trees for 5 OTUs is 15 (Option 2).

Correct option: (2) 15