Paramecium is a unicellular ciliate protozoa that has two nuclei. One is a macronucleus, which is responsible for controlling the everyday functions of the cell, such as metabolism and protein synthesis. The other is a micronucleus, which contains the genetic material necessary for sexual reproduction.

The macronucleus is polyploid and contains multiple copies of chromosomes, while the micronucleus is diploid and contains one copy of each chromosome. During sexual reproduction, the micronucleus undergoes meiosis to produce haploid nuclei, which then fuse to form a zygote.

Therefore, the correct answer is paramecium.