What causes some flowers, like Hibiscus, to close?

The correct answer is 'At sunset'.

Many flowers, including Hibiscus, have a biological mechanism that causes their petals to open and close in response to changes in light and temperature. When the flower is exposed to sunlight, a hormone called auxin is produced that triggers the growth and expansion of cells in the outer petals. This causes the flower to open. However, when the light fades or temperatures drop, the production of auxin decreases, causing the cells in the outer petals to shrink and the flower to close. This mechanism is an adaptation that helps to protect the reproductive structures of the flower from damage and conserve energy during periods when there is little or no light available. While there may be other factors that can cause flowers to close, such as drought, disease, or physical damage, the primary cause of the opening and closing of flowers like Hibiscus is a change in light and temperature.