Question 1

1. a cannot be derived, each identifier has a minimal length of two characters as neither <identifier> nor <subsequent identifier> are able to produce the empty string.
2. a2

   <identifier>
   ---> <initial> <subsequent identifier>
   ---> <letter> <subsequent identifier>
   ---> a <subsequent identifier>
   ---> a <subsequent>
   ---> a <digit>
   ---> a2
   

3. 2a cannot be derived as <initial> does not produce digits.
4. ?a?

   <identifier>
   ---> <initial> <subsequent identifier>
   ---> <special initial> <subsequent identifier>
   ---> ? <subsequent identifier>
   ---> ? <subsequent> <subsequent identifier>
   ---> ? <initial> <subsequent identifier>
   ---> ? <letter> <subsequent identifier>
   ---> ?a <subsequent identifier>
   ---> ?a <subsequent>
   ---> ?a <initial>
   ---> ?a <special initial>
   ---> ?a?
   

5. @a@ cannot be derived as <initial> does not produce @.

Question 2

<List> ---> ( <ListOfElements> )
<ListOfElements> ---> <empty> | <CommaSeparatedList>
<CommaSeparatedList> ---> <Element> | <Element> , <CommaSeparatedList>
<Element> ---> <List> | <Digits>
<Digits> ---> <Digit> | <Digit> <Digits>
<Digit> ---> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9