555 timer operating modes: which of the following correctly describes the set of timing/multivibrator modes that a standard 555 timer IC can be configured to operate in using appropriate external components?

Introduction / Context:
The ubiquitous 555 timer is a flexible analog IC used for pulse generation, oscillation, and delays. It is often introduced in early electronics courses to demonstrate timing networks and comparator-based control. Knowing its supported modes is necessary to select the right external resistor-capacitor (RC) network and wiring.

Given Data / Assumptions:

• Standard bipolar or CMOS 555 timer.
• External R and C determine timing.
• Configuration is chosen via how threshold, trigger, and discharge pins are interconnected.

Concept / Approach:
The 555 readily supports astable (free-running oscillator) and monostable (one-shot pulse) configurations with minimal external parts. Although a 555 can be wired as a bistable latch using its reset and threshold/trigger comparators, the canonical textbook “two primary modes” are astable and monostable, and many question banks focus on that pair. Hence the best answer among the provided choices is the option that includes astable and monostable together.

Step-by-Step Solution:

Verification / Alternative check:
Reference schematics show the discharge transistor alternately pulling the timing capacitor to ground (astable) or providing a single discharge path after a trigger (monostable). Lab kits and datasheets emphasize these two modes prominently.

Why Other Options Are Wrong:

• Astable only / Monostable only: incomplete—555 does both.
• Bistable multivibrator: possible with special wiring, but not the “paired” answer offered here; the two-mode choice is the accepted response in standard sets.
• Crystal-stabilized oscillator only: the 555 is not a crystal-controlled oscillator.

Common Pitfalls:
Assuming the 555 is limited to a single mode; overlooking reset pin behavior; ignoring duty-cycle control nuances in astable mode.

Final Answer:
Astable multivibrator and a monostable multivibrator