When H forms a bond with H2O to form the hydronium ion H3O , this bond is called a coordinate covalent bond because

both bonding electrons come from the oxygen atom

Explanation:

A coordinate covalent bond is said to be formed when an electron pair is donated into a empty orbital of an atom from a filled orbital of another atom.

In this type of covalent bond, the shared electron pair is furnished by only one of the bonding species.

The bonding between H^+ and H2O to form H3O^+ involves donation of a lone pair of electrons on oxygen into the empty 1s orbital of  H^+ hence the two bonding electrons come from oxygen. This is why the bond is called a coordinate covalent bond.

answer: option (b) is the correct answer.

explanation:

a covalent bond is formed by sharing of electrons. whereas polarity arises due to difference in electronegativity of two atoms.

therefore, more is the electronegativity of an atom which combines with the titanium, more readily it will induce a partial positive and negative charge.

thus, out of the given options fluorine is most electronegative and has an incomplete octet. therefore, it will readily combine with titanium to share the electrons and complete its octet.  

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.