Which Type of Mutation Is Likely to Be Associated with a Milder

Which type of mutation is likely to be associated with a milder osteogenesis imperfecta phenotype?

A) Missense mutation for glycine in pro $\alpha$ ₁(1) chain
B) Missense mutation for glycine in pro $\alpha$ ₁ (1) ) chain
C) Missense mutation in either chain disrupting rate of $\alpha$ helix formation
D) Mutation affecting posttranslational modification of collagen
E) Null mutation for pro $\alpha$ ₁(1) chain