Please help with that Mensa Romania IQ test question. I think it's B, but why?
The answer is
The third column shows the lines that are different between the first two columns. That is, if a line segment appears in both the first two columns, then it will not appear in the third; likewise if a line segment appears in neither of the first two columns it will not appear in the third. However any line segment that appears in exactly one of the first two columns will be present in the third column.
In a computer-science context, you might say that the columns are related by exclusive-or (XOR) - applying that operation to any two columns will yield the other.