The question says this is
an upper air contour chart, which gives the height of upper pressure levels
derived from both msl pressure and airmass temperature.
The tag "London"
tells you it is northern hemisphere and the wind arrow tells you that the low
contours are on the left of the arrow (Buys Ballot's law)
If you track back along the contours you find A is on a higher contour line
than B, so true height, which is what contours show, is higher at A.
Flying "on the contour chart" means you are flying at a constant
pressure level, 500mb, 300mb or whatever the chart is drawn for.
The altimeter setting
given, 1013.2, is irrelevant. If you fly at a constant pressure setting,
whatever it is, toward low contours you are going down, toward high contours,
up.