The blank requires a verb at its place as it is preceded by the infinitive marker 'to' which requires a verb in its base form after it. The word 'uncover' fits correctly in the blank as its use conveys that the scientists have been unable to figure out what causes Alzheimer's disease and that is holding out the ongoing research. Hence, option (a) is the correct answer.
The words 'decodes', 'solves' and 'decrypted' are not the base forms of verbs. Hence, options (b), (d) & (e) are eliminated. The word 'interpretation' is a noun, but the blank requires a verb. Hence, option (c) is eliminated.
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