Hi Solen
It seems that you are not attempting dynamic table selections... Why not try one of the following options:
1) Make LT_MARA a Class Attribute Like Umit suggested. Or
2) You could define your method as:
Methods: get_data EXPORTING et_tab TYPE STANDARD TABLE.
ZCL_TEST_PERFORMANCE=>GET_DATA( IMPORTING et_tab = <LFS_TAB> ).
Hope it helps
Regards
Vic