Which of the following molecules contains a covalent bond?

  m

Explanation:

in chemistry, a solution’s concentration is how much of a dissolvable substance, known as a solute, is mixed with another substance, called the solvent. the standard formula is c = m/v, where c is the concentration, m is the mass of the solute dissolved, and v is the total volume

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)