Final answer:
In Beethoven's String Quartet in C Minor, the melody in the opening is played by the violin, and each time it returns, it is varied somehow. The tempo of the movement is Allegro, and the piece is primarily in the key of C minor but may include modulations.
Step-by-step explanation:
In Beethoven's String Quartet in C Minor, Op. 18, No. 4, IV, the melody in the opening section is played by the violin. This quartet, which is scored for two violins, viola, and cello, does not include a flute. Therefore, the answer to the question about which instrument plays the melody is the violin (a).
The opening section returns several times in the movement and each time it returns, it is varied in some way (c). This could include slight alterations in melody, rhythm, harmony, or dynamics. As for the tempo of this movement, the term Allegro (a) best describes it, indicating a brisk and lively speed.
According to the title of the work, and the common practice in classical compositions, this work is indeed in the key of C minor (c); however, like many classical pieces, it likely modulates or shifts between C minor and C major (b), providing contrast and interest throughout the piece.