Which of the Following Techniques Exemplifies Phenotypic Rescue in a Host

Which of the following techniques exemplifies phenotypic rescue in a host cell?

A) Introducing a genomic library from a cell that produces a particular gene product into a mutant host cell auxotroph that cannot synthesize the product.
B) Introducing a genomic library from a mutant cell auxotroph for a particular gene product into a host cell that can synthesize that product constitutively.
C) Removing genes that encode a particular gene product from a cell that expresses that product constitutively before creating a genomic library.
D) Replacing a mutant gene with a functional gene in a DNA library from an auxotrophic mutant cell prior to introducing the library into a related host cell.