Answer
a) $0\;\rm rad$
b) $4\pi\;\rm rad$
c) $\pi\;\rm rad$
Work Step by Step
a) We can see, from the given graph, that points A and B are at the front of the same wave.
So,
$$\boxed{\Delta \phi_{(A|B)}=\color{red}{0}\;\rm rad}$$
b) $$\Delta \phi_{(C|D)}=2\pi\dfrac{\Delta r}{\lambda}$$
where $\Delta r$ here is about $2\lambda$
$$\Delta \phi_{(C|D)}=2\pi\dfrac{2\lambda}{\lambda}$$
$$\boxed{\Delta \phi_{(C|D)}=\color{red}{4\pi}\;\rm rad} $$
c) $$\Delta \phi_{(E|F)}=2\pi\dfrac{\Delta r}{\lambda}$$
where $\Delta r=\frac{1}{2}\lambda$
$$\Delta \phi_{(E|F)}=2\pi\dfrac{\frac{1}{2} \lambda}{\lambda}$$
$$\boxed{\Delta \phi_{(C|D)}=\color{red}{\pi}\;\rm rad} $$