The question asks for the lowest available flight level. Some basics - the transition altitude outside CAS is 3,000ft. The first fl30 on that track is FL30 and it is also the transiton level. The transition level is not available for use as it does not garauntee any separation from aircraft flying at the transition altitude. Therefore in this case FL50 is the first available FL on that track that provides any separation from the transition altitude
That is wrong.
There is no UK requirement for an aircraft crusing at the transition level to be separated from another aircraft at the transition altitude.
The quadrantal rule does not provide any separation. It is simply an orderly system that can remove some possible conflictions but you can till have an aircraft approaching you at the same level..........from behind, from the left from the right and also from directly ahead! - and that is just the IFR ones who are complying with the quadrantal rule. Add in the VFR flights who ignore the rule and you will see that quadrantal rule and separation should not be used in the same sentence.
The answer is FL30 as this is the first flight level available for use above the transition altitude of 3000ft.
Regards,
DFC