In digital communications, modulation where the two binary values (0 and 1) are represented by two distinct amplitudes of the same carrier frequency is called:

Introduction / Context:
Digital modulation maps bits to changes in a carrier signal so information can be sent reliably over band-limited, noisy channels. A foundational scheme alters only the amplitude of a fixed-frequency carrier to distinguish binary symbols. This question asks you to identify the precise name of that method.

Given Data / Assumptions:

• Binary symbols must be encoded on a single carrier frequency.
• Only the carrier amplitude changes between the two bit values.
• Phase and frequency remain constant across symbols.

Concept / Approach:
When binary data is conveyed by toggling the carrier's amplitude between two discrete levels—one for bit 0 and another for bit 1—the scheme is called amplitude-shift keying (ASK). If the lower level is exactly zero for one symbol, it is sometimes referred to as on-off keying (OOK), which is a special case of ASK. This differs from amplitude modulation (AM), an analog scheme where a continuous message waveform varies the carrier amplitude across a continuum of values, not two discrete levels. It also differs from frequency-shift keying (FSK) and phase-shift keying (PSK), which vary frequency or phase, respectively.

Step-by-Step Solution:

Verification / Alternative check:
Textbook signal constellations show ASK with two points along the amplitude axis at the same phase angle. Practical radio, optical, and RFID links often employ OOK (a form of ASK) for simplicity in low-complexity devices.

Why Other Options Are Wrong:

• Amplitude: not a modulation method by itself.
• Amplitude modulation: analog (continuous) rather than binary symbol mapping.
• Aloha: a multiple-access protocol, not a modulation type.
• None of the above: incorrect because ASK fits exactly.

Common Pitfalls:
Confusing analog AM with digital ASK; assuming OOK is separate rather than a subset of ASK; overlooking that noise immunity is typically better with PSK/QAM than simple ASK in many channels.

Final Answer:
Amplitude-shift keying.