"The sandwich has meat in it."
Translation:La sandviĉo havas viandon en si.
You are right, and I agree that “havas viandon en si” should be accepted.
Maybe the author of this sentence was thinking about an implied relative clause such as “…viandon, kiu estas en ĝi.” But I think “si” is the correct form. Many speakers would avoid the dilemma by saying “enhavas viandon” (which NicolasDaL698501 proposed, and which is also accepted).
The version with “si” is now the proposed translation.
As Skidbladner wrote, “si” is the reflexive pronoun. To make that clear it might be translated here by “itself” instead of “it.” “Si” is used instead of “li,” “ŝi,” “ĝi” or “ili” (or words derived from them) whenever they refer back to the subject of the sentence:
- Sofia parolas al si = Sofia is talking to herself
- Sofia parolas al ŝi = Sofia is talking to her (somebody else)