Help for my etrog plant
According to our check, Etrog is a kind of citrus. (If this is another species, please let us know.) Citrus in general can be a bit challenging indoors. (We have some information on this here: https://extension.umd.edu/hgic/topics/growing-dwarf-citrus.) They prefer very good light (bright light for as much of the day as possible), humid air, cool temperatures (especially overnight), and soil that is allowed to get slightly dry between waterings. Leaf loss in plants can point to multiple issues, but cultural (in other words, not insect or disease) conditions are typically the first to stress a plant. In the case of inadequate humidity, using a room humidifier may help.
If a plant isn't able to nutritionally support all of its leaves - either through sufficient moisture from the roots or via enough light to maintain photosynthesis - it will shed those it cannot support in order to conserve resources. Indoor plants are commonly grown in locations with insufficient light. Our eyes are very good at adapting to different light levels, so it can be difficult to determine how bright or dim (from a plant's perspective) an area really is. Try moving this plant to a bright window; if that window receives direct sunlight, use a sheer curtain or other fine material to lightly shade it from the brightest light for its first week to give it time to adjust. If it burns or drops the final leaf, give it some time, as new ones adapted to this new light level may grow.
Second, be sure that the soil isn't staying waterlogged. Overly-wet soils don't allow enough gas exchange for the roots to breathe, and they can die or succumb to fungal rot. Once this happens, it is difficult to correct; giving the plant fresh soil and more time between waterings is the only recourse if the damage isn't too pervasive. Its pot should have drainage holes (given the saucer you're using, we are guessing it does). The saucer should not collect water for long, as this will re-absorb into the soil and keep conditions too wet.
The pot might be too big for the plant, depending on how many roots it has. Many tropical plants we grow tend to do well being slightly rootbound (cramped in their pot), partly as this avoids the risk of over-watering. In pots that are too large, the risk is from the root-less soil retaining water for too long.
Do not fertilize, as this may exacerbate its problems. If a nutrient deficiency does appear in the leaves, first make sure the roots seem intact and healthy, as often an insufficient root system is what is really responsible for malnourished foliage. (The nutrients may be in the soil, but there aren't enough roots to adequately absorb them.) Soils should be slightly acidic for roots to pick up the nutrients they need; if this is old potting soil, its pH could have slowly changed outside of the ideal range. Old potting soil also compresses more as it ages, creating conditions that promote root rot. Repotting is typically done on houseplants in spring, but any time poor soil or root issues are suspected, it can be done at any time.