Final answer:
Marla is using social comparison with peers to assess her writing ability by comparing her grades to Jenna's, which is a common part of identity formation in adolescence.
Step-by-step explanation:
When Marla, age 14, decides that she is a good writer because she gets better grades than her friend Jenna on stories she writes for English, she is demonstrating the use of social comparison with peers as part of identity formation. This concept explains how individuals determine their own social and personal worth based on how they stack up against others they perceive as somehow faring better or worse. Marla is using her peer, Jenna, as a benchmark to assess her own abilities in writing, which is a common way adolescents gauge their talents and form their identity.