This is part of the Operational Procedures syllabus and refers to a technique now thankfully lost in the mists of time.
ISO means 'the same as' or 'equal', CHRONOS means 'time' so an ISOCHRONE is a line of equal time.
The simplest way to illustrate this is with a series of diagrams, for simplicity I've put the expanation in a PDF file
here
.... and I think the answer is (d), though you could argue you pass not from the air isochrone but from the preceding ground isochrone to the origin for the next isochrone by moving from K to K'.
Can you believe this rubbish?