In industrial control, a programmable controller (e.g., a PLC) is designed for reuse and field flexibility; it can be reprogrammed at the site and can be reassigned across different machines during its service life.

Introduction / Context:
Programmable logic controllers (PLCs) and similar controllers are purpose-built for industrial environments. Their core value proposition is flexibility: engineers can download new ladder logic, function block diagrams, or structured text to adapt the same hardware to new tasks, often without removing it from the machine or panel.

Given Data / Assumptions:

• Industrial processes change; control logic must be updated.
• Controllers support on-site reprogramming over serial/Ethernet connections.
• Hardware can be redeployed to different machines when needs shift.

Concept / Approach:
Separation of hardware I/O and software logic enables reuse. Field reprogramming is standard and supported by vendor tools, versioning, and backups. While training is advisable for safety and quality, the requirement for “considerable training before any use” is not universally true and depends on task complexity, not on the controller’s intrinsic nature.

Step-by-Step Solution:
Check portability: I/O cards and CPUs can be reassigned.Check flexibility: logic can be edited and downloaded on site.Select the combined option reflecting both reuse and field reprogramming.

Verification / Alternative check:
Maintenance teams frequently adjust timing, interlocks, and sequences during commissioning or product changeovers via field edits, confirming both properties.

Why Other Options Are Wrong:
(a) only or (b) only: each is true but incomplete alone.Requires considerable training: depends on task; not a defining property.None of the above: incorrect because (a) and (b) are correct together.

Common Pitfalls:
Editing logic without change control; insufficient backups; and failing to coordinate changes with safety interlocks and lockout/tagout procedures.

Final Answer:
(a) and (b) above