Which level of processing should result in the longest lasting memory codes?

The answer would be semantic encoding

There are several encoding types like s tructural encoding, mnemonic encoding, semantic encoding, phonemic encoding, visual encoding, and acoustic encoding.

From these types, semantic encoding is proven to last longer in the memory. In semantic encoding, the input is applied into a context. The process will involve many areas of the brain, makes it easier to recall and last longer.