For the TTL logic family, the approximate output levels for logic 0 and logic…

Concept: A logic family's output levels are fixed by what its output stage can physically produce, not by the input thresholds that a gate is guaranteed to recognise. In a standard TTL totem-pole output stage the HIGH level is the supply rail reduced by the forward drops of the pull-up transistor and its series diode, while the LOW level is the collector-emitter saturation voltage of the pull-down transistor.

Applying it here:

1. Standard TTL is powered from a single positive supply VCC of about 5 V referred to ground, so every output level must lie between 0 V and 5 V.

2. Driving logic 1 (HIGH): the totem-pole pull-up transistor and the series diode each drop a forward voltage of about 0.7-0.8 V, so the output settles roughly 1.4-1.6 V below VCC, that is near 3.4-3.5 V.

3. Driving logic 0 (LOW): the pull-down transistor is driven into saturation and holds the output at its VCE(sat), which datasheets put at roughly 0.2 V typical and 0.4 V maximum while it sinks rated load current.

4. So a TTL output swings between a low level of a few tenths of a volt and a high level near 3.5 V; of the pairs offered, 0.6 V and 3.5 V is the one that matches, with 0.6 V a loose approximation of the low level.

Cross-check against the other values:

• Output levels are not the same as input thresholds. The familiar 0.8 V LOW-maximum and 2.0 V HIGH-minimum figures describe what a TTL input is guaranteed to recognise; a pair that clears them is readable by the next gate, but it still does not say what the driving stage can actually deliver, which is what this question asks for.

• 5 V is VCC itself. Because of the two forward drops in the step above, a totem-pole output cannot rise to the full rail, and 0.1 V is below the saturation voltage that a loaded pull-down transistor develops.

• 0.9 V and 1.75 V both fall in the indeterminate band between the 0.8 V and 2.0 V input thresholds.

• -1.75 V is an ECL swing level; ECL operates from a supply below ground, whereas TTL has no negative output level.

Result: the approximate TTL output levels are about 0.6 V for logic 0 and about 3.5 V for logic 1.