A. Genomic DNA.
Eucaryotic promoters can be isolated from a genomic DNA library.
A genomic DNA library is a collection of cloned DNA fragments that represent the entire genome of an organism. It contains fragments of genomic DNA, including both coding and non-coding regions, and can be used to isolate specific DNA sequences, including eukaryotic promoters.
Promoters are DNA sequences located upstream of genes that play a crucial role in initiating gene transcription. To isolate eukaryotic promoters, researchers can screen a genomic DNA library using specific probes or by performing functional assays that identify regions with promoter activity.
Option B, cosmid cDNA library, refers to a library constructed using cosmid vectors, which can accommodate larger DNA fragments. While cosmid cDNA libraries can be used for various purposes, they are not specifically designed for isolating promoters.
Option C, phage lambda cDNA library, refers to a library constructed using lambda phage vectors to clone cDNA (complementary DNA) fragments. Phage lambda cDNA libraries are commonly used for the cloning and characterization of expressed genes, but they are not the primary choice for isolating eukaryotic promoters.
Therefore, the correct answer is A. Genomic DNA.