Hand contacting op amp photodiode circuit causes interference - what ist the physic behind it?
Your option b is correct.

Your body is mostly insulated from ground, so it acts like a plate of a capacitor. The other plate is the rest of the world, or better, the portion of world somewhat near you.

A part of this other plate is, for example, the mains wires in the ceiling or walls. Since the voltage in them oscillates at 50 Hz, and you and them make a capacitor, then some of this voltage is present also on your body. When you touch a high impedance node, such as the cathode of the diode, this voltage is injected in the circuit, amplified and you can see it on the output.

Try to touch the circuit ground node as well, and you will see the noise decrease dramatically.

And use a shielded cable, not a ribbon cable, or even better put the amplifier next to the diode.