Which research method involves an in-depth investigation of an individual subject?

The correct answer is case study. 

A case study is a method of psychological research that involves collecting in-depth information and data about an individual person or animal. Information gathered from case studies can be from a combination of questionnaires, interviews, observations etc. Case studies can be conducted when investigating any psychological phenomena, however, they are most useful when investigating psychological or mental disorders.

The will be a : great britain wanted french gold mines

Texas’ gulf coastal plains are the western extension of the coastal plain extending from the atlantic ocean to beyond the rio grande. its characteristic rolling to hilly surface covered with a heavy growth of pine and hardwoods extends into east texas. in the increasingly arid west, however, its forests become secondary in nature, consisting largely of post oaks and, farther west, prairies and brushlands.