Introduction:
During a menstrual cycle, there are several phases that occur in the female reproductive system. These phases include the menstrual phase, follicular phase, ovulation, and luteal phase. The timing of copulation during these phases can greatly affect the probability of fertilizing an egg. In this case, the question asks which phase has the least probability of fertilization.

Explanation:
To determine which phase has the least probability of fertilization, let's analyze each phase and its characteristics:

1. Menstrual Phase:
During this phase, the endometrium, the lining of the uterus, sheds off along with blood. It is the first phase of the menstrual cycle and generally lasts for 3-5 days. The menstrual phase is characterized by low hormone levels and the absence of a mature egg. Therefore, copulation during this phase has no chance of fertilizing an egg.

2. Follicular Phase:
The follicular phase is the phase leading up to ovulation. It starts on the first day of menstruation and lasts until ovulation occurs. During this phase, follicle-stimulating hormone (FSH) is released, stimulating the development of several follicles in the ovaries. These follicles produce estrogen, which helps thicken the uterine lining. As the follicles develop, one dominant follicle emerges and continues to mature. Copulation at the end of the follicular phase has a higher probability of fertilizing an egg.

3. Ovulation:
Ovulation is the release of a mature egg from the ovary. It usually occurs around day 14 of a 28-day menstrual cycle. Ovulation is triggered by a surge in luteinizing hormone (LH), which causes the dominant follicle to rupture and release the egg. Copulation during ovulation has the highest probability of fertilizing an egg since the egg is present and viable in the fallopian tube.

4. Luteal Phase:
After ovulation, the ruptured follicle forms a structure called the corpus luteum, which secretes progesterone. The luteal phase begins after ovulation and lasts until the start of the next menstrual cycle. During this phase, the uterine lining thickens further to prepare for implantation. If fertilization does not occur, hormone levels decrease, and the uterine lining sheds off during the next menstrual phase. Copulation at the end of the luteal phase has a lower probability of fertilization since the egg has already been released and is no longer viable.

Conclusion:
Based on the characteristics of the menstrual cycle, copulation at the end of the follicular phase has the least probability of fertilizing an egg. This is because the egg has not been released yet, and it is not present in the fallopian tube. It is important to note that the timing of copulation relative to ovulation greatly influences the chances of fertilization and conception.