An example of a public key encryption algorithm is:
(A) Caesar cipher algorithm
(B) DES algorithm
(C) AES algorithm
(D) Knapsack algorithm
D is the answer
The First General Public-Key Algorithm used what we call the Knapsack Algorithm. Although we now know that this algorithm is not secure we can use it to look at how these types of encryption mechanisms work.
For details see https://nrich.maths.org/2199