PC-to-mainframe connectivity: the software that allows a personal computer to behave like a character or block-mode terminal (for example, VT100 or 3270) is called:

Introduction / Context:
Before pervasive web clients and thin-client agents, users commonly accessed host systems through terminal protocols. To connect using a PC, software was needed that replicated the behavior and control sequences of dedicated terminals so the host believed it was talking to the expected device type.

Given Data / Assumptions:

• Goal: make a PC act like a particular terminal model and protocol.
• Use cases include mainframes, minicomputers, and network devices.
• Connectivity may occur over serial lines, Telnet, or SSH.

Concept / Approach:
Terminal emulation software interprets and generates the same control codes, screen layouts, and key mappings as the target terminal (e.g., ANSI/VT100, IBM 3270/5250). It provides the user interface and protocol handling so legacy hosts function correctly without physical terminals.

Step-by-Step Solution:

Verification / Alternative check:
Common programs such as PuTTY (VT100/VT220 emulation) or tn3270 clients demonstrate this concept in modern environments.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing transport (modems/TCP) with presentation (terminal type and control sequences).

Final Answer:
terminal emulation.