Final answer:
Freud would attribute a child's regression to thumb sucking and mouth touching to a fixation in the oral stage of psychosexual development. This stage focuses on oral pleasures, and fixations may lead to behaviors like smoking or nail-biting in adulthood.
Step-by-step explanation:
According to Sigmund Freud, if a child reverts back to behaviors such as thumb sucking and constant touching of their mouth, Freud would say that the child was fixated in the oral stage of psychosexual development. The oral stage is the first of Freud's developmental stages, occurring from birth to about 1 year. In this stage, the main source of pleasure is the mouth, as seen in activities like eating and sucking. If a child does not receive proper nurture during this stage, or if the weaning process is handled inappropriately, Freud suggested they might develop an oral fixation later in life, resulting in behaviors such as smoking, drinking, overeating, or nail-biting as a way to ease anxiety.