Final answer:
The metric used to estimate effort in Planning Poker is the Fibonacci sequence. This sequence helps teams in agile software development provide relative estimates for tasks, promoting discussion and consensus.
Step-by-step explanation:
The metric used to estimate effort in Agile development during Planning Poker is c) The Fibonacci sequence (1, 2, 3, 5, 8, 13, 21, 34, etc.). Planning Poker is a consensus-based technique designed to help agile teams estimate the effort or complexity of work items. Team members use cards with Fibonacci numbers to represent their estimates, with each number signifying a relative estimate of effort, complexity, and uncertainty for a given task. Choosing from a sequence such as the Fibonacci sequence helps avoid the false precision of linear scales and encourages team members to discuss significant differences in estimation.
As for the provided reference information, it does not directly relate to Planning Poker, but it exemplifies the process of estimation, scale, and measurement in different contexts. Planning Poker, however, is specifically focused on software development tasks and not on physical measurements like the length or size of objects or distances.