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