To form a meaningful sentence, the parts must describe the Moon's origin. Starting with c, "the origin of the Moon," as the subject, followed by b, "was long a mystery, and," sets the context. Then, a, "beginning in the 19th century," indicates the timeframe, and d, "three competing theories emerged to solve it," concludes with the resolution.
The sentence is: "The origin of the Moon was long a mystery, and beginning in the 19th century, three competing theories emerged to solve it."
Option 4 (cbad) is correct.
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