Whilst the correct answer is arrived at, I would dispute a couple of the statements in G-CEXO's explanation.
The Transition Level is more usually defined as the first 500ft level above the Transition Altitude; it is not a usable cruising level as it will not provide adequate separation from an aircraft cruising at the Transition Altitude. Consequently, whilst the Transition Level in this example is, indeed, FL45, the lowest available cruising level is FL50. This would be significant if the aircraft's track was north westerly as the minimum quadrantal level would be FL65 and not FL45.
The Transition Layer is the vertical distance between the Transition Altitude and the Transition Level which, in the example given, is 210ft
-----------Transition Level------------------ 4500ft 1013mb - FL45
Transition Layer (4500-4290=210ft)
----------Transition Altitude----------------- 3000ft 970mb (4290ft 1013mb)
^
3000ft
v
----------------MSL------------------------- 0ft 970mb (1290ft 1013mb)
^
1290ft
v
------------------------------------------- -1290ft 970mb (0ft 1013mb)