This question is a little confusing. Good questions should not be confusing. Good questions should let the student have clarity on what is being asked and which opiton is undoubtely the right answer.
Let me tell you why I say this.
A line (-) is used to represent pairs of share electrons and that is a covalent bond.
So the line is used when you have covalent bonds. Is that the answer? It should be but when study the other options you find another option which is a good fit.
A line is used in the structural formula to represent the covalent bonds, i.e. the pairs of shared electrons.
So, the line is used to represent covalent bonds (option b) in a structural formula (option q)
This is an example:
H
|
H - C - H
|
H
That is the structural formula of CH4 and the lines represent the covalent bonds (pairs of shared electrons).
If you have to pick one, it sounds better the option q. (only because syntaxis), i.e.
A structural formula uses lines to indicate pairs of shared electrons in a molecule.
Because it is not exact that the covalent bonds uses lines ... but that lines are used to represent covalent bonds.
Conclusion: the option q.
