What is the approximate bandwidth of a 13-WPM International Morse Code transmission?
Why For on-off keyed CW the bandwidth depends on the keying speed, not on the transmit frequency. The standard estimate is bandwidth = baud rate times a fading factor K, where K is about 5 for hand-sent Morse. A 13 WPM signal has a keying rate of roughly 10.4 baud (about 0.8 times the words per minute), so 10.4 x 5 gives approximately 52 Hz.
Watch out The choices equal to the WPM number or twice it come from just reading the speed off the question; they ignore the fading factor of 5 that the standard CW bandwidth estimate applies.
CW bandwidth is roughly 4 times the WPM: 13 WPM x 4 = 52 Hz.