My thought process when doing this question:
-I treated all the elements as if they belonged to period 3.
-I referred to data booklet’s 2nd IE to obtain Al’s 2nd IE for rough guage (1820)
-Looked at the graph and thought H was Ar and worked backwards to get C which I thought was Al.
could you tell me where I went wrong? Or which steps
For such questions, you're supposed to figure out the answer by pattern recognition, rather than using actual IE values from Data Booklet (which should be done only as a last resort, if you've no idea how to proceed without such).
If the graph was for 1st IE, then G would be Group 18 (period 3). But since the graph is for 2nd IE, then G must be Group 1 (period 4), hence working backwards, A is Group 13, ie. Al.