The probability of fertilizing an egg during copulation varies throughout the menstrual cycle. The menstrual cycle consists of four main phases: menstrual phase, follicular phase, ovulatory phase, and luteal phase.

Least probability of fertilization during copulation
Among these phases, copulation during the end of the follicular phase has the least probability of fertilizing an egg. This is because during the follicular phase, the ovaries are preparing to release an egg, but the egg has not been released yet. Therefore, there is no egg available for fertilization yet.

Follicular phase
The follicular phase is the first phase of the menstrual cycle and lasts for about 10 to 14 days. During this phase, the pituitary gland releases follicle-stimulating hormone (FSH) which stimulates the growth of follicles in the ovaries. Each follicle contains an immature egg. As the follicles grow, they produce estrogen, which thickens the uterine lining in preparation for a potential pregnancy.

Ovulatory phase
The ovulatory phase occurs mid-cycle and is characterized by the release of a mature egg from one of the follicles. This process is triggered by a surge of luteinizing hormone (LH) from the pituitary gland. The mature egg then travels down the fallopian tube, where it may be fertilized by sperm.

Luteal phase
The luteal phase occurs after ovulation and lasts for about 12 to 16 days. The empty follicle from which the egg was released transforms into a structure called the corpus luteum, which produces progesterone. Progesterone helps to maintain the thickened uterine lining in preparation for a potential pregnancy. If the egg is not fertilized, the corpus luteum breaks down and progesterone levels decrease, leading to the shedding of the uterine lining and the start of a new menstrual cycle.

In conclusion, copulation during the end of the follicular phase has the least probability of fertilizing an egg since the egg has not been released from the follicle yet.