The correct answer is: credible.
A credible long-term strategy is one that is believable and likely to be successful. It is based on sound research and analysis, and it is supported by evidence. A credible long-term strategy is essential for any organization that wants to achieve its goals.
The other options are not as relevant to the question. A contestable strategy is one that is open to debate or challenge. A creditable strategy is one that is worthy of praise or admiration. A compatible strategy is one that is consistent with or in harmony with another strategy. A conceivable strategy is one that is possible or imaginable.
In the context of the question, the opposition’s effort of unity is frustrated by the absence of a credible long-term strategy. This means that the opposition does not have a plan that is believable and likely to be successful. Without a credible long-term strategy, the opposition is unlikely to be able to achieve its goals.