Confusion with flux and Lenz's law in an LC tank circuit
Can we say that at point b, the net magnetic flux through the inductor is maximum?


Yes


  But in this case the magnetic field already is created by the coil it is not external. So does the Lenz law apply here?


Lenz Law does not state that the magnetic field should be external. In fact any change in the magnetic field will give rise to EMF which will try to oppose the change in magnetic field. This is actually how an inductor works, if the current through the inductor changes it will create a potential difference across its ends which would try to oppose the increase in current and hence increase in the magnetic flux (due to its own current) through the inductor coils or vice versa.
In fact this is what is called self inductance, that is, opposing the change in current due to its own magnetic flux. There is another type of inductance called mutual inductance which comes into play when there are two or more coils which couple their magnetic fields. In this case (in addition voltage due to self inductance) there will be additional potential difference to oppose the changing magnetic fields of the nearby coils.