Which of the databases are you likely to use for research topics related to…

Concept: Abstracting and indexing databases are discipline-specific. Each one is compiled by a body that serves a single field of study and indexes only the journals, conference papers and reports of that field. So choosing a database for any research topic is a matching exercise: identify the discipline the topic belongs to, then pick the database whose declared subject scope is that discipline.

Application: The topic here belongs to biology, i.e. the life sciences. Among the databases listed, PubMed is the one whose subject scope is biomedicine and the life sciences: it is maintained by the National Center for Biotechnology Information (NCBI) at the US National Library of Medicine and provides access to the MEDLINE citation set covering molecular biology, genetics, physiology, microbiology and clinical medicine. A search for a biology research topic is therefore run in PubMed.

Cross-check: Mapping every listed database to the discipline its producer declares shows that only one of them serves the life sciences:

Engineering, geoscience and mathematics indexes do not cover the life-science journals a biology topic needs, so the database to use is PubMed.