The correct answer is option 4: silica.

Silica is added to cement to lower the setting time. When silica is present in the cement mixture, it reacts with the calcium hydroxide released during the hydration process of cement. This reaction forms additional compounds, such as calcium silicate hydrate, that contribute to the hardening of the cement.

The addition of silica ensures a more rapid formation of these compounds, which leads to a faster setting time for the cement. This is particularly useful in construction projects where time is a critical factor, as it allows for quicker curing and early strength development.

It is important to note that the other options mentioned in the question, such as oxides of aluminium, gypsum, and oxides of magnesium, do not directly lower the setting time of cement. These materials may have other beneficial effects on cement properties, but they do not have a significant impact on reducing the setting time.