The correct answer is option 'B' - Culture medium.

Explanation:

In vitro fertilization (IVF) is a technique used to assist couples who are unable to conceive naturally. It involves the fusion of an ovum (egg) and sperm outside the woman's body, in a laboratory setting.

IVF procedure involves several steps, and the fusion of ovum and sperm occurs in a culture medium. Let's understand the process in detail:

1. Ovarian Stimulation:
- Prior to IVF, the woman undergoes hormonal stimulation to produce multiple eggs instead of the usual single egg that is released during a normal menstrual cycle.
- This is done by administering fertility drugs that stimulate the ovaries to produce more follicles, each containing an egg.

2. Egg Retrieval:
- Once the eggs have sufficiently matured, they are retrieved from the woman's ovaries using a minor surgical procedure called follicular aspiration.
- This procedure is performed under ultrasound guidance, and a thin needle is used to extract the eggs from the follicles.

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

4. Insemination:
- The retrieved eggs and the sperm sample are then brought together in a laboratory dish containing a culture medium.
- The culture medium provides the necessary nutrients and conditions to support the fertilization process.
- The eggs and sperm are incubated together for a specific period of time, allowing fertilization to occur.

5. Embryo Development:
- After fertilization, the resulting embryos are monitored for development in the culture medium.
- The embryos are kept in the laboratory for a few days to allow them to develop into a more advanced stage.

6. Embryo Transfer:
- Once the embryos have reached a suitable stage of development, one or more embryos are selected for transfer into the woman's uterus.
- The embryos are carefully placed into the uterus using a thin catheter, inserted through the cervix.

7. Implantation and Pregnancy:
- If implantation occurs, the transferred embryo(s) will attach to the uterine lining, leading to pregnancy.

In summary, the fusion of the ovum and sperm occurs in a culture medium during the insemination step of the IVF procedure. The culture medium provides the necessary environment for the sperm to reach and fertilize the eggs, mimicking the natural conditions inside the fallopian tubes.