The correct answer is A. Hydrogen.
Hydrogen is the lightest element in the periodic table, with an atomic number of 1 and an atomic mass of 1.00794 u. It is a colorless, odorless, and tasteless gas that is the most abundant element in the universe. Hydrogen is the simplest element and is composed of a single proton and a single electron. It is a non-metal and is highly flammable.
Helium is the second lightest element in the periodic table, with an atomic number of 2 and an atomic mass of 4.002602 u. It is a colorless, odorless, and tasteless gas that is the second most abundant element in the universe. Helium is a noble gas and is
chemically inert.Oxygen is the third lightest element in the periodic table, with an atomic number of 8 and an atomic mass of 15.9994 u. It is a colorless, odorless, and tasteless gas that is essential for life. Oxygen is a non-metal and is highly reactive.
Carbon is the fourth lightest element in the periodic table, with an atomic number of 6 and an atomic mass of 12.011 u. It is a non-metal and is the basis for all organic compounds. Carbon is highly versatile and can form a wide variety of compounds.