According to an agile iterative development process in which iteration would implementation activities normally start?

It will begin on the First iteration. Scrum and agile are both incremental and iterative. They are iterative in that they get ready for crafted by one cycle to be enhanced in ensuing emphasises. They are incremental on the grounds that finished work is conveyed all through the undertaking. 
An iterative procedure is a procedure for figuring the coveted outcome by methods for a rehashed cycle of operations. An iterative procedure ought to be united, i.e., it should come nearer to the coveted outcome as the quantity of cycles increments.

Sector is best defined as (part).