In "packed one", 'one' is a pronoun that refers to "piglet". It isn't specified which packet, hence the indefinite article 'a' is the most appropriate option.
'A' and 'an' are indefinite articles which modify nouns/pronouns that are not specific or particular. ('A' is used when the noun/pronoun begins with a consonant and an is used when it begins with a vowel. Even though 'one' begins with a vowel, it is pronounced with a 'w' sound: won, hence 'a' will be used). 'The' is a definite article used when the noun/pronoun is specific.
Since the noun 'one' is general, it is not a particular packed one, option C is correct and A and B are incorrect.
'Some' as a determiner is used for plural or uncountable nouns. Since "packed one" is singular and countable (as it is one); option D is incorrect.