We used to have a definition of horizontal surface visibility of:
'a darker object visible against a lighter background'. Of course the visibility we're talking about here is in-flight vis, which tends to be less than surface vis.
Commercial aviation moved on decades ago to 'Runway Visual Range' which was determined by counting runway lights from a fixed position then ATC would convert this into RVR. At civilised 'dromes like London we had caravans but at Eastleigh they used to use a step-ladder. It's all been replaced by transmissometers now (used to be yellow, the latest kit all seems to be red, dunno why.)
Anyway, determining 1,500m airborne is really tricky, as soon as you've seen something at the edge of visibility, it's under you and there's something else appeared out of the gloom. Personally, there's NO WAY I'd go flying VFR in vis. that poor, but the OP was asking about exams, not reality...
TOO