Используйте слово SEE в правильной форме: When the macaques caught____of their own reflection,they were fascinated.

When the macaques caught the seeing of their own reflection, they were fascinated. - Когда макаки увидели собственное отражение, они были очарованы.

The seeing is the gerund, which combines the characteristics of a verb and a noun. Past Simple active (caught) and Past Simple passive (were fascinated) are used in the sentence.