The frequency of the lowest standing wave refers to the fundamental frequency.
For fundamental frequency, the formula is:
f = v/(2L) where f is the fundamental frequency
v is speed of the wave
L is the length of the string.
To determine the speed of the wave, substitute values of f and L to the formula.
f = v/(2L)
40 = v/(2*1)
40 = v/2
v = 80
The speed of the wave is 80 m/s.
Answer: (d) 80 m/s