Hormone secretion is often regulated through a negative feedback mechanism, which means that once a hormone is secreted it will cause the hypothalamus and pituitary to shut down the production of signals necessary to secrete the hormone in the first place. most oral contraceptives are made of small doses of estrogen and/or progesterone. why would this be an effective means of contraception?

Oral contraceptives inhibit the secretion of gonadotropins by inhibiting the secretion of GnRH through a negative feedback mechanism.

Explanation:

Most oral contraceptive pills have combined dosages of estrogen and progesterone or one of these hormones. The estrogen and progesterone are involved in negative feedback inhibition of gonadotropin-releasing hormone (GnRH). Presence of small dosages of estrogen and progesterone inhibit the secretion of GnRH and thereby, prevents the secretion of gonadotropins (LH and FSH).  

FSH (Follicle-Stimulating Hormone) is required for maturation of follicles while LH (Luteinizing Hormone) is required for ovulation of mature follicle.  

Hence, hormonal contraceptives prevent conception by preventing follicle development and ovulation due to the absence of gonadotropins (FSH and LH).  

the last one: artificial selection/genetic engineering

plant breeding is modifying the genetic composition of the plant - but this isn't cloning (which is specifically introducing a gene into a genome). but you are artificially selecting for the phenotype that you want.

the second one is literally genetic engineering - the gene would have been cloned first and then inserted or genetically engineered into the new plant cell (the cotton).