Which Tolkien work, published in 1977, was edited and completed by his son Christopher?

The Tolkien work published in 1977 and edited and completed by his son Christopher is The Silmarillion.

J.R.R. Tolkien worked on the mythology later known as The Silmarillion throughout much of his life, but he did not finish it in a final publishable form before his death in 1973. Christopher Tolkien organized a large body of drafts, revisions, notes, and variant texts and prepared the posthumous book for publication.

The Silmarillion provides the cosmological and historical background to The Hobbit and The Lord of the Rings. It includes the creation story of Eä, the early history of the Elves, the wars against Morgoth, and accounts of figures such as Fëanor, Beren, Lúthien, and Túrin Turambar. Because Tolkien repeatedly revised the material, Christopher had to make editorial decisions when arranging it. The book is therefore not simply a manuscript reproduced unchanged, but an edited presentation of Tolkien’s extensive legendarium.