The formal phonological rule $"$ V $-->$ V : $/$

C $[+$voice$]"$ is best translated as

Vowels become nasalized in the environment where they come before a

consonant.

Vowels become lengthened in the environment where they come before a

voiced consonant

Vowels become nasalized in the environment where they come after a

consonant

Vowels become lengthened in the environment where they come after a voiced

consonant