Main group elements are the elements which belong to the s and p block of the periodic table:
Here in the question ,
• Cs is in group IA elements ( belong to the s block; therefore main group)
,
• Br in in group 4( belong to the s,p block, therefore main group)
,
• Al is in group 3 (belong to the first group of p-block elements, therefore main group)
,
• ,Mo is in group 6, but in the d block , therefore not in the main group.
,
• Pb (copper) is a d block element and therefore not in the main group.,
The answer are therefore; Pb and Mo