According to the classical Indian school of logic (Nyaya), which of the following steps of inference involves the synthesis of the major term (sadhya), the middle term (hetu) and the minor term (paksa)?

"Heraclitus is a man who is invariably a mortal" fuses all three terms.

This is the upanaya (application) step of the Nyaya five-membered inference.

Paksa (minor, Heraclitus), hetu (middle, man) and sadhya (major, mortal) meet here.

It applies the universal rule of the example to the case at hand.

The synthesis of minor, middle and major terms is what upanaya performs.