Mark thinks that language development over the life span requires a slow but steady shaping process. His belief is most directly relevant to the issue of

A) continuity or stages.
B) secure or insecure attachments.
C) fluid or crystallized intelligence.
D) cross-sectional or longitudinal studies.

Mark's belief that language development requires a slow but steady shaping process is related to the issue of continuity or stages. This is because the continuous view of development suggests that development is a gradual and continuous process, while the stage view suggests that development occurs in distinct stages. Mark's belief in a slow and steady shaping process suggests a gradual and continuous approach to language development, which aligns with the continuous view of development.