Two homologous chromosomes paired together, each of which has two sister chromatids.
Explanation:
Zygotene stage of prophase I of meiosis I is characterized by the pairing of homologous chromosomes with help of synaptonemal complex. Since DNA replication has occurred in S phase, each chromosome in the pair contains two sister chromatids.
So, the chromosome pair appears to have a total of four chromatids and the structure is termed as "tetrad".
