• this is incorrect The full adder for n-bit number requires n full adder cascaded ....... it should say The parallel adder for n-bit number requires n full adders cascaded – jsotola Oct 8 '18 at 16:48
• Ok...! So you mean to say that full adder is just a basic circuit that is used to add multi-bit but when adding more than 1-bit number, its circuit is known as parallel adder? – Naveed Baig Jokhio Oct 8 '18 at 17:08

• a full adder adds two bit plus a carry in bit (in contrast to a half adder, which adds two bits only)

• a parallel adder has adder circuits (full adders) for each pair of bits (in contrast to a serial adder, which has just one (full) adder circuit and uses a shift register to add each bit pair in sequence)

• a ripple-carry adder is the simple form of a parallel adder, where the carry-out of each full adder is connected to the carry-in of the next full adder. Hence the total delay time of the adder is the time it would take for a carry to ripple through all bit-pair full adders, as for 1111 + 0001. (in contrast to various carry-lookeahd generation circuits that can be used to reduce this delay time, at the cost of extra circuitry).

So you confusion is caused by the three terms you mention to be characterizations that address different levels of adder circuits.