Which of the following is NOT a standard synchronous data link protocol? (Choose the data-link option that is specifically asynchronous or otherwise not a synchronous link protocol.)

Introduction / Context:
Data link protocols can be broadly categorized as synchronous (timing derived from a shared clock; frames sent in fixed timing) or asynchronous (characters framed with start/stop bits without a shared clock). Recognizing these categories is essential in legacy WAN and serial communication contexts.

Given Data / Assumptions:

• We are comparing data-link protocols by their timing method.
• Focus is on whether a protocol is a standard synchronous link protocol.
• We will select the option that is specifically not synchronous at the data-link level.

Concept / Approach:
SDLC (Synchronous Data Link Control) is, by definition, synchronous. SLIP (Serial Line Internet Protocol) encapsulates IP over serial lines using asynchronous character framing (start/stop bits) and lacks features like error detection and addressing—it is not a synchronous data link protocol. SMTP is an application-layer email transport and not a data-link protocol at all, but the prompt directs us to pick the data-link option that is not synchronous.

Step-by-Step Solution:
Identify synchronous vs asynchronous: SDLC → synchronous; SLIP → asynchronous.Exclude non–data-link distractors (SMTP, PAS unspecified) per the instruction to choose the data-link option.Select SLIP as the data-link mechanism that is not synchronous.

Verification / Alternative check:
SLIP historically runs over RS-232 lines using asynchronous framing; PPP later replaced SLIP with richer features (LCP, authentication) but remained compatible with asynchronous serial operation.

Why Other Options Are Wrong:
SDLC: Canonical synchronous link protocol.

SMTP: Application-layer email protocol; not a link protocol (outside the comparison domain).

PAS: Not a standard data-link protocol; distractor.

None of the above: Incorrect because SLIP clearly is not synchronous.

Common Pitfalls:
Confusing OSI layers; comparing application protocols with link protocols leads to category errors. The safest approach is to restrict the comparison to data-link candidates.

Final Answer:
SLIP