Which of the following wave has the largest wave speed?

A wire of length 0.4 m stretched at both ends vibrates 250 times per second. If the length of the wire is increased by 0.1 m and the stretching force is reduced to $1 / 4$ th of its original value, then the new frequency is

Two strings $A$ and $B$ produce beat of frequency $\Delta f_1>0$. The tension in string $A$ is slightly increased and the beat frequency is found to be $\Delta f_2>0$. If the original frequency of $A$ is $f_0$ and $\Delta f_2<\Delta f_1$, then the frequency of $B$ is

The distance between two successive minima of a transverse wave is 2.7 m . Five crests of the wave pass a given point along the direction of travel every 15.0 s . The speed of the wave is