Output voltage offset in op amp
Sorry if I wasn't clear.  In part C, I hope you put a 100K resistor to ground on the positive input, so that the effect of Ib is cancelled; so the offset current is left (10na).  The capacitor blocks DC, so the current has to flow through the feedback resistor.  10nA * 100K = 1mv.  Because bias current flowing out of each input is the same, and the DC impedance becomes identical when you add the resistor, the bias current component is cancelled out and lacking offset current you would have zero at the output.  So the offset current component appears across one of the 100 k resistors but not the other.  Hence 10nA * 100K = 1mv.