Hydrogen bond is formed when positive end of one molecule attracted negative end of other molecule. The concept is similar to magnetic attraction where opposite poles attract each other.
Explanation:
While understanding hydrogen bond, two terms are important, one is electronegativity (tendency of atoms to attract electrons towards itself) and other is dipole (separation of positive and negative charge in a molecule). Hydrogen bond is always formed between hydrogen atom and other atoms having electronegativity different than hydrogen.
Differ from Ionic and Covalent Bonds:
Hydrogen bond is weaker than ionic and covalent bond. Ionic and covalent bonds are intramolecular (within the molecule) whereas hydrogen bond is intermolecular (between molecules).
The answer is Strontium(Sr). The reactive increase from right to left. And this element has two valence electrons. So Rb is not correct. Then the very reactive metal is Sr.