Although none of these are completely correct, I'd choose D. to change tenses, an irregular verb has to change its spelling.
A is definitely incorrect - that is a rule about regular, rather than irregular verbs. B is also incorrect - there are no rules about irregular verbs so you have to learn them all by heart. C is incorrect because this is true for regular verbs as well, not just irregular.
This leaves us with D as the correct answer. Often, various forms of irregular verbs change more or less drastically. For example: see - saw - seen; ring - rang - rung, etc.
