The sentence is about contrasting the nature of a masterpiece with chance or raw talent.
Starting with C — "a laborious career than as the lucky fluke of" — sets up a comparison, implying that a masterpiece is more the result of hard work than mere chance.
Next, D — "more likely to come as the culminating point of" — continues logically from C, explaining the conditions under which a masterpiece emerges.
Following D, A — "but a masterpiece is" — shifts the focus to explicitly defining what a masterpiece actually is, providing a contrast to the earlier part.
Finally, B — "untaught genius" — completes the definition by stating that a masterpiece is not merely the product of raw, untrained talent.
Thus, the meaningful sequence is C → D → A → B, which matches option (a) CDAB.