An iron nails sinks in water, while a large ship floats on the surfacr of water

I don't exactly understand the substance of the question so im going to assume that the answer you are looking for is buoyancy. Buoyancy determines whether something sinks or floats in water. A ship has a larger buoyancy due to the shape of the hull, a nail is does not and will sink.

probably not if net force is zero

hi there!

out of the two, (b) precision would be more reasonable.   however, both a and b could be pretty reasonable.

-awesomerepublic   : )