The preferred site is the male pronucleus, so the correct answer is (2) Male pronuclei.

Option-by-option explanation

(1) Female pronuclei

• The female pronucleus is present, but it is smaller and less distinct under the microscope.

• Technically DNA can integrate if injected here, but it is not preferred because it is harder to target reliably and the success rate is lower.

(2) Male pronuclei – correct

• After fertilization but before syngamy, the sperm-derived male pronucleus is typically larger and more clearly visible than the female pronucleus.

• This makes it the standard target for pronuclear microinjection when generating transgenic mice and many other animals.

• Injecting here ensures the transgene is present very early, so it can integrate before the first cleavage and be carried into all cells, including germ cells.

(3) Cytoplasm

• Cytoplasmic injection leaves DNA outside the nucleus initially; integration into chromosomes is inefficient and less predictable.

• This method is not the classical approach for making stable, germline transgenic animals.

(4) Cleavage cells

• Injecting at later cleavage stages (e.g., 2‑cell or 4‑cell embryos) often produces mosaic animals, because only some blastomeres receive and integrate the DNA.

• Mosaicism reduces the chance that germ cells carry the transgene, so cleavage cells are not preferred.

Hence, for producing transgenic animals from fertilized eggs, the male pronucleus is the preferred site for transgene insertion.