Answer: Option (a) is the correct answer.
Explanation:
The given data is as follows.
  mass per unit length ([tex]\mu[/tex]) = [tex]\frac{M}{l}[/tex] = 1 kg/m
   Tension = 4 N
  Speed (v) = [tex]\sqrt{\frac{F}{\mu}}[/tex]
So, when F is doubled then change in value of F will be as follows.
        F = 4 + 4 = 8 N
Therefore, speed will be calculated as follows.
      v = [tex]\sqrt{\frac{8 N}{1 kg/m}}[/tex]
       = 2.8 m/s
Thus, we can conclude that the wave speed is 2.8 m/s.
