If nothing is mentioned take fan-in for a logic gate = 2 inputs and they will say in the question whether to take half adder or full adder for the 1st stage of LSB addition(ripple carry adder).
Propagation delay In carry look-ahead adder(n bit adder):-
Carry look-ahead adder works in three phases:-
In ripple carry adder all Pi, Gi terms are computed simultaneously after that all carry terms are computed simultaneously and after that all sum terms are computed simultaneously.
The first level will take 2 time unit to generate all Pi, Gi terms using AND-OR gate in 2 levels only if variables are given in both complemented and uncomplemented form.
similarly 2nd level will also take further 2 time unit for generating the all carry terms.
and after that to produce the sum terms it will take further 2 time unit. so total propagation delay is 6 time unit.
Note:- Any simple or complex logical function can be written in SUM OF PRODUCT form and can be realized using 2-levels of AND, OR gate only if both complemented and uncomplemented forms are given.
Now lets extend this question further for Ripple carry adder:-
In ripple carry adder(n bit adder):- Assume Full adder is used for LSB computation, not half adder
Tpropagation =(n-1)* T.carry generation time by one full adder+ max(T.carry, T.sum)
= 3 * 2 (2 because C=AB+BC+AC can be implemented in 2 levels of AND, OR gate) + max(2, 4)
T.sum=4, because A⊕B can be implemented using 2 levels of AND, OR gate only if variables are given in both complemented and uncomplemented form as mentioned above. then further one more XOR with C. so total 4 level of AND, OR gate.
Tpropagation for ripple carry adder = 10