Решение:

Решение через if:

Program Case1;
Var n: integer;
Begin
     Read(n);
     Case n of
          1,2,12 : Write('зима');
          3..5 : Write('весна');
          6..8 : Write('лето');
          9..11 : Write('осень');
          else write('error');
     end;
End.

Program If1;
Var n: integer;
Begin
     Read(n);
     If (n=1) or (n=2) or (n=12)
     then Write('зима')
     else If (n>=3) and (n<=5)
             then Write('весна')
             else If (n>=6) and (n<=8)
                     then Write('лето')
                     else If (n>=9) and (n<=11)
                             then Write('осень')
                             else write('error');
End.