The National Center for Biotechnology Information (NCBI) hosts a variety of important biological databases and tools.
(a) GenBank: A primary nucleotide sequence database containing publicly available DNA sequences.
(b) PubMed: A free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. It provides access to biomedical literature, including journal articles, and links to full-text content where available (e.g., via PubMed Central).
(c) UniProt (Universal Protein Resource): A comprehensive database of protein sequence and functional information. It's a collaboration between Swiss Institute of Bioinformatics (SIB), European Bioinformatics Institute (EBI), and Protein Information Resource (PIR). NCBI also has its own protein database.
(d) BLAST (Basic Local Alignment Search Tool): A suite of programs used to compare biological sequences (DNA or protein) against sequence databases to find regions of similarity. It's a search tool for sequences, not primarily for literature.
Therefore, PubMed is the NCBI database (or more accurately, a search interface to databases like MEDLINE) used for searching and retrieving biomedical literature.
\[ \boxed{\text{PubMed}} \]