Despite its initial _____, the theory proved remarkably robust when subjected to rigorous experimental verification, defying early predictions of its imminent collapse.

✓ Correct Answer: Option B

The sentence uses 'despite' to indicate a contrast. The theory was 'remarkably robust,' so its initial state must have been the opposite, suggesting 'fragility' or weakness, making its robustness surprising. 'Complexity,' 'novelty,' and 'ubiquity' do not create this contrast.