Treatment objective for serial extraction:: (A) To intercept a developing arch-length deficiency and to reduce or eliminate the need for extensive appliance therapy, (B) Reduce arch-length deficiency, (C) To plan for extensive appliance therapy, (D) To reduce arch-length deficiency & to plan for extensive appliance therapy
Answer is A. Benefits of Serial Extraction 

Serial extraction guides or encourages eruption of permanent teeth in a favorable position.
Reduces malposition of individual teeth.
Avoids loss of labial alveolar bone.
Reduces treatment time when active orthodontic treatment is required. 

Textbook of ORTHODONTICS Sridhar Premkumar