In reply to Juhi_Patel:
Reason is your statement
c_inst[i]= new();
There is no element of array c_inst yet .
Change your pre_randomize() to ::
function void pre_randomize();
c_inst = new[10]; // 10 elements at default value 'null'
foreach(c_inst[i])
begin
c_inst[i]= new();
c_inst[i].randomize(); // Not Needed !!
end
endfunction