For designing a tractor, the assembly line production would entail deciding the sequence of various operations, place for men, machines and raw materials etc. All these are part of:

Option 1: Motion study -> Focuses on analyzing individual body movements of workers to eliminate unnecessary motions and improve efficiency of specific tasks.

Option 2: Work measurement -> Concerned with determining the time required to complete a specific job, establishing time standards for operations.

Option 3: Method study -> Systematic examination of work processes to develop better methods, including sequence of operations, workplace layout, and positioning of men, machines, and materials.

Option 4: Time study -> A technique for recording and analyzing the time taken to perform specific job elements under defined conditions.

Hence, Option 3: Method study -> Method study is the systematic recording and critical examination of ways of doing work to develop improved methods. It specifically deals with deciding the sequence of operations, workplace layout, and optimal placement of workers, machines, and raw materials in production systems like assembly lines. This makes it the correct answer for designing a tractor assembly line production system. -> correct