Explanation

What we know:
--- L has 10 well-known plays with comments, including Medea
--- L has 8 without comments. The 8 without only appear in L (Electra is one of them)
--- the 10 well-known plays also appear in other manuscripts

A. Only Euripides’ best-known works are accompanied by ancient commentaries in extant medieval manuscripts.
We don't know whether any other manuscripts besides L have commentary to be with. "Only" is an extreme word.

B. The select plays are accompanied by ancient commentaries because they were the best known of Euripides’ works.
We don't know if popularity was the driving force for the commentary. Maybe they just have the most relevant subject matter to the people commenting or something.

C. No commentaries were written about Euripides’ Electra in ancient times.
We don't know anything about commentaries other than that L has some. It's entirely plausible that some were written but are now lost. "No" is an extreme word.

D. Euripides’s Medea never appears in medieval manuscripts unaccompanied by ancient commentary.
As in C, we don't know anything about commentaries other than that L has some. "Never" is an extreme word.

E. Euripides’ Electra does not appear accompanied by a commentary in any extant medieval manuscript.
We know this because Electra ONLY appears in L, and is one of the 8 that does not have commentary.

Answer: E