Explanation

At the standard MP3 bitrate of 128 kbps, each minute of music occupies about 0.96 MB (128,000 bits/s x 60 s / 8). Sixty minutes therefore need roughly 57.6 MB, which is closest to the 50 MB option. 600 MB and 200 MB are far too large, while 10 MB would imply only about 22 kbps, below acceptable music quality. Note: UPSC's published key marks option C (10 MB), but the bitrate arithmetic clearly supports about 50 MB.