How to properly terminate this circuit (50ohm coaxial to oscilloscope)
Option (a) is the normal approach because: -


The op-amp output sees the least loading effect due to the 50 ohms (R7) being in series with the output.
The voltage seen at the scope-end is ideally the same voltage as delivered by the op-amp (after a delay of course).


Option (b) just shunts the very low output impedance of the op-amp with 50 ohms i.e. it is ineffectual.

In option (c) R8 can be used (compared to option (a)) but it usually doesn't bring a whole lot to the party other than halving the signal amplitude.

Option (d) loads the op-amp with 50 ohms and this might be a little too "heavy handed" for most op-amps.