In Paramecium, both autogamy and conjugation are called sexual reproduction because of

The correct option is "the fusion of haploid gametes".

Understanding Sexual Reproduction in Paramecium:
In Paramecium, both conjugation and autogamy are considered processes of sexual reproduction.
Let us break down why this is the case:

1. Conjugation: This is a temporary union of two individuals (conjugants) during which they exchange genetic material. Specifically, the diploid micronucleus of each conjugant undergoes meiosis to form haploid pronuclei (which act as gamete nuclei). One of these haploid pronuclei from each individual migrates and fuses with the stationary haploid pronucleus of the other partner, forming a diploid zygote nucleus.
2. Autogamy: This is a process of self-fertilization occurring within a single individual. Similar to conjugation, the diploid micronucleus undergoes meiosis to form haploid nuclei. Two of these haploid nuclei (representing male and female gamete nuclei) then fuse together within the same cell to form a diploid zygote nucleus (synkaryon).

Conclusion:
Although conjugation involves two individuals and autogamy occurs within a single individual, the fundamental feature that makes both of them forms of sexual reproduction is the production of haploid gametic nuclei (gametes) and their subsequent fusion to form a diploid zygote. Thus, they are classified as sexual reproduction because of the fusion of haploid gametes.