The yet-to-be born mammalian offspring following the embryonic stage, and is still going through further development prior to birth. Supplement. Following the embryonic stage, the developing young enters the fetal period, which is in the later stages of development prior to birth.

The correct answer is option 'A': Well-developed embryo.

To understand why the correct answer is option 'A', let's first define what a foetus is and then discuss the other options.

What is a foetus?
A foetus is the stage of development in mammals, including humans, after the embryonic stage and before birth. It is the period during which the structures and organs of the organism continue to grow and mature.

Options:

a) Well-developed embryo:
This option is correct. A foetus is a well-developed embryo that has reached a stage of advanced development. At this stage, most of the major organs and systems have formed, and the foetus is capable of independent survival outside the womb, although it is still dependent on the mother for nutrition and protection.

b) Developing embryo:
This option is incorrect. A developing embryo refers to the earlier stage of development before the foetus stage. During this stage, the embryo undergoes rapid cell division and differentiation to form the basic structures and organs of the organism.

c) A zygote:
This option is incorrect. A zygote is the fertilized egg formed by the fusion of a sperm and an egg. It is the earliest stage of development and undergoes multiple cell divisions to form an embryo.

d) Male gamete:
This option is incorrect. A male gamete refers to a sperm cell. Sperm cells are responsible for fertilizing the female egg, resulting in the formation of a zygote.

Conclusion:
In conclusion, a foetus is a well-developed embryo that has reached an advanced stage of development. It is the stage before birth where most of the major organs and systems have formed. The other options (b, c, and d) are incorrect as they refer to earlier stages of development or different reproductive cells.