(a) simple graph with 1.5x the frequency
(b) Graphs are based on the Doppler equation. The graph given in the problem is for a moving observer.
Which is based on
As the observer’s velocity increases, the frequency increases linearly with it as is shown in the problem
The new graph is based on a source moving towards you.
As can be seed from this equation, as the source increases velocity, the frequency increases but when the source approaches the speed of sound, the frequency approaches ∞ and becomes undefined so has a limit to it unlike in the first graph.