Final answer:
The term for giving girls below the age of 18 for marriage is Child marriage. It's often driven by socio-cultural and economic factors and is associated with various health and education issues.
Step-by-step explanation:
The term referring to the practice of giving girls below the age of 18 years old for marriage is D. Child marriage. This practice involves young girls, often from lower socio-economic backgrounds, who are married off to older men before they reach adulthood. The underlying factors can include gender roles, socio-cultural traditions, and economic reasons, such as the expectation of a dowry. Child marriage is linked with various other issues, including teen pregnancy, lack of education, and health concerns like female genital mutilation (FGM).