Which material is filled inside fluorescent lamp?

The correct answer is B: Mercury 

Mercury is a chemical element with the symbol Hg and atomic number 80. It is commonly known as quicksilver and was formerly named hydrargyrum.

Mercury is essential to a fluorescent bulb's ability to emit light; no other element has proved as efficient. As effective as it is at enabling white light, however, mercury—sometimes called quicksilver—is also highly toxic. It is especially harmful to the brains of both fetuses and children.Compact fluorescent light bulbs (CFLs) use dramatically less energy than incandescent bulbs and reduce greenhouse gas emissions. But they also contain mercury – a dangerous toxin. ... If a CFL breaks, some of the mercury that's contained in the bulb will evaporate into the air.

An average of 4 milligrams (mg) of mercury is in a CFL. A mercury thermometer may contain about 500 mg of mercury. About 500 mg to 700 mg or less of mercury is considered a small spill. Some CFLs may contain as little as 1.4 mg to 2.5 mg per light bulb.