If you have a look at experimental data (e.g. https://www.osti.gov/servlets/purl/6548367/ and https://www.preprints.org/manuscript/201610.0095/v1/download) you will find that min drag for a symmetric aerofoil tends to be a little bit past zero incidence.

What I suggest is that the way the question is styled reflects this. The only sensible answer is (b). The pedants might opine for (d), again as one can find data to support that option. However, I suspect the examiner will be looking for (b)

There are plenty of papers available on the net for you to delve into to get a handle on what's going on. You need to keep in mind that pilot theory training endeavours to simplify stuff to make the general understanding a bit easier. Unfortunately, sometimes that can confuse the issue a bit if you look into things a bit deeper. I'd be a bit careful making statements like "as at 4 degree drag will be lowest" .. I can pull out experimental data which shows quite different numbers. Again, just one of the problems of trying to simplify and generalise stuff which should have lots of qualifications and caveats ...