The correct answer is option 'D', culture medium.

IVF (In vitro fertilization) is a technique used to help individuals or couples who are struggling with infertility to conceive a child. It involves the fusion of an ovum (egg) and sperm outside of the human body, in a laboratory setting. The fusion of ovum and sperm, known as fertilization, occurs in a culture medium in IVF.

Here is a detailed explanation of how the fusion of ovum and sperm occurs in the IVF technique:

1. Ovarian Stimulation: The IVF process begins with ovarian stimulation, where fertility medications are administered to the woman to stimulate the ovaries to produce multiple eggs. This is done to increase the chances of successful fertilization.

2. Egg Retrieval: Once the eggs have matured, they are retrieved from the woman's ovaries using a minor surgical procedure called transvaginal ultrasound-guided follicle aspiration. This procedure is performed under sedation or anesthesia.

3. Sperm Collection: On the same day as the egg retrieval, the male partner provides a semen sample. If the male partner has fertility issues, sperm can also be obtained through other methods like testicular sperm extraction (TESE) or epididymal sperm aspiration (PESA).

4. Insemination: In the laboratory, the retrieved eggs are then prepared for insemination. The eggs are placed in a culture dish containing a culture medium. The culture medium is a nutrient-rich fluid that provides an optimal environment for the eggs and sperm.

5. Fertilization: The sperm is then added to the culture dish containing the eggs, and the dish is kept in an incubator. The sperm naturally swim towards the eggs, and if all goes well, fertilization occurs. The sperm penetrates the outer layer of the egg, and the genetic material from the sperm combines with the genetic material from the egg, resulting in the formation of an embryo.

6. Embryo Development: The fertilized eggs, now embryos, are monitored in the laboratory for several days. The embryos undergo cell division and development. The embryologists carefully observe the embryos to select the healthiest and most viable ones for transfer.

7. Embryo Transfer: After a few days, typically 3-5 days after fertilization, the selected embryos are transferred into the woman's uterus. This procedure is performed by passing a thin catheter through the cervix and into the uterus. The embryos are carefully placed in the uterus, where they can hopefully implant and result in a pregnancy.

In summary, the fusion of ovum and sperm in the IVF technique occurs in a culture medium, which provides the necessary nutrients and environment for successful fertilization to take place. The culture medium mimics the natural conditions found in the fallopian tube, where fertilization normally occurs in natural conception.