To solve this problem, we need to identify which organism exhibits isogamy with non-flagellated gametes.$\newline$Let's analyze each option:$\newline \bullet$Option 1: Sargassum$\newline$ - Sargassum is a genus of brown algae. It typically exhibits oogamy, where there is a large non-motile egg and a smaller motile sperm.$\newline \bullet$Option 2: Ectocarpus$\newline$ - Ectocarpus is also a brown alga. It shows anisogamy, where gametes are of different sizes but both are motile.$\newline \bullet$Option 3: Ulothrix$\newline$ - Ulothrix is a green alga that exhibits isogamy, but the gametes are flagellated and motile.$\newline \bullet$Option 4: Spirogyra$\newline$ - Spirogyra is a filamentous green alga that exhibits isogamy with non-flagellated gametes.$\newline$ - In Spirogyra, gametes are similar in size and shape, and they fuse to form a zygospore without the need for flagella.$\newline$Therefore, the correct option is Option 4: Spirogyra, which shows isogamy with non-flagellated gametes.