Which method of contraception involves inserting a substance into the vagina using a syringe?

The method of contraception that involves inserting a substance into the vagina using a syringe is foam. This type of contraception typically refers to a spermicide, which is a chemical that immobilizes or kills sperm to prevent fertilization. When administering foam spermicide, it is inserted into the vagina with a syringe or applicator, creating a barrier against sperm entering the cervix.

Foam spermicide is effective when used correctly and can enhance the effectiveness of other contraception methods, like condoms, by providing an additional layer of protection against pregnancy. This is distinct from other methods listed, such as the birth control pill, which is taken orally and works systemically to prevent ovulation, or condoms, which are worn externally to create a barrier against sperm. The IUD, on the other hand, is a physical device inserted by a healthcare provider into the uterus, not through the vagina with a syringe. Therefore, foam stands out as the correct choice for this question.