For the two similar voting groups to gain power, it would make sense to:

The passage states that the greens and democratic socialists vote in unison, and the conservative and social conservatives also vote similarly. This means that these two groups have a combined total of 4 voting groups. Libertarians, on the other hand, tend to vote on an issue-by-issue basis, so they are not always predictable.
By creating an alliance with the libertarians, the two similar voting groups would be able to increase their chances of gaining power. The libertarians would be more likely to vote with these two groups on issues that they agree on, and this would give the combined group a majority of the votes.
The other options are not as likely to be successful. Adding a sixth political party would simply dilute the power of the existing parties. Combining the conservative parties into a single party would also dilute their power, and it is not clear that the combined party would be more likely to gain the support of the libertarians. Having fewer votes would obviously make it more difficult for any group to gain power.
Therefore, the best way for the two similar voting groups to gain power is to create an alliance with the libertarians.