We aren't able to identify the type of citrus if the plant wasn't labeled, but the picture looks like it's showing a spent flower that may have been pollinated. Time will tell if it develops into a viable fruit; it's too early to be certain, and it's normal for a citrus plant to drop most of its immature fruits (at least, when multiple fruits are forming simultaneously) as a way to conserve energy. The citrus flower's ovary (the structure that will develop into the edible fruit with seeds inside) lies between and somewhat above the petals (rather than behind them), so as the petals wither away and fall off, the green nodule left behind is the ovary. If it remains and gradually gets larger, then it is developing into a fruit, though immature fruits can be shed early and not ripen. Causes can include insufficient pollination or stress on the plant, like insufficient light, roots that get too wet or too dry, hot or cold drafts, or other changes in environmental conditions.

Our Growing Dwarf Citrus page has some cultivation information, though we don't have care and troubleshooting details beyond what is included in that resource. Key to good growth are bright light (including putting the plant outdoors for the summer, if you have the space), moderate humidity levels (especially indoors in winter), and avoiding over-watering by letting the soil get fairly dry to the touch an inch or more deep into the container before watering again.

Miri