Conjugation as a mode of reproduction is present in

The correct option is ciliata.

Let us break down the details of conjugation and why it is present in ciliates:

1. What is Conjugation?
Conjugation is a sexual process in which two unicellular organisms or single cells temporarily pair up and fuse to exchange genetic (nuclear) material. Unlike complex sexual reproduction, it does not involve the formation and fusion of distinct gametes, but it achieves the same goal of genetic recombination.

2. Occurrence in Protozoans
Protozoans are classified into different groups based on their mode of locomotion:
- Flagellata (Mastigophora): Move using flagella (e.g., Euglena, Trypanosoma). They primarily reproduce asexually by longitudinal binary fission.
- Sarcodina (Rhizopoda): Move using pseudopodia (e.g., Amoeba). They reproduce asexually by binary fission or multiple fission.
- Sporozoa: Spore-forming parasitic protozoans (e.g., Plasmodium). They reproduce through complex asexual (schizogony) and sexual (sporogony) phases, but do not undergo conjugation.
- Ciliata: Move using cilia (e.g., Paramecium). They possess nuclear dimorphism (a large vegetative macronucleus and a small reproductive micronucleus) and characteristically reproduce sexually via conjugation.

3. How it Works in Ciliates
During conjugation in ciliates like Paramecium:
- Two compatible individuals (conjugants) come together and adhere at their oral grooves.
- The macronucleus degenerates, while the micronucleus undergoes meiosis to produce haploid pronuclei.
- The conjugants exchange haploid micronuclei, which fuse to form a diploid zygotic nucleus.
- The cells then separate (now called exconjugants) and undergo division to form new individuals.

Thus, conjugation as a mode of reproduction (and genetic rejuvenation) is a defining feature of the group Ciliata.