Any combination of unique letters, numbers, or blanks, which describes and identifies any activity or task shown on the schedule, is:

An Activity ID (Identifier) is a unique alphanumeric code assigned to each activity within a project schedule. Its purpose is to uniquely identify and differentiate each task, preventing confusion and ensuring accurate tracking, sequencing, and resource allocation. The combination of letters, numbers, or symbols is structured to be unique for every activity, directly aligning with the definition provided in the question. This identifier is fundamental to the logical structure and management of the project schedule network.

B. WBS Code: This code identifies elements within the Work Breakdown Structure (WBS), which are deliverable-oriented groupings of work, not the individual schedule activities themselves.
C. Resource: A resource is the labor, equipment, or material required to perform an activity. It is not the identifier for the activity.
D. Account number: This is a code from the Chart of Accounts used for cost tracking and management. While linked to activities, its primary function is financial, not schedule identification.

1. AACE International, Recommended Practice No. 10S-90, Cost Engineering Terminology, (June 28, 2019).

ACTIVITY IDENTIFIER (ID): (p. 9) "The unique alphanumeric code assigned to each activity in the network." This definition directly matches the question's description.

WORK BREAKDOWN STRUCTURE (WBS): (p. 133) Defines the WBS as a "product-oriented family tree subdivision" of project work, confirming that its codes apply to work packages, not individual activities.

RESOURCE: (p. 104) Defined as "labor, equipment, materials, and money that are required for project execution," not as an identifier.

CODE OF ACCOUNTS (COA): (p. 30) Described as a "systematic coding structure for organizing and managing resource, cost, and schedule activity information," highlighting its primary role in cost management rather than as a unique activity identifier for scheduling logic.

2. Hollmann, J. K., Editor (2015). AACE International's Skills & Knowledge of Cost Engineering (6th ed.). AACE International.

Section 7.3.2.1 Activity Definition: (p. 7.3.4) This section discusses the process of identifying and documenting the specific activities that must be performed. It states, "Each activity is assigned a unique activity code or ID number." This reinforces that the Activity ID is the specific term for the unique identifier of a schedule task.