Write down what you are given
∠APQ=43∘ Angle APQ is 43°. The question is which angle fact takes you from there to the angle that was written down.
Look at the first step, which is not the one being asked about
∠APQ=43∘ The first step uses a different fact (alternate angles are equal) to get angle APQ = 43°. The question asks about the SECOND step only.
Describe where angle APQ and angle BPE sit
∠APQ=43∘,∠BPE=43∘ Angle APQ and angle BPE are opposite each other at the point where two straight lines cross.
Name the fact
Vertically opposite angles are equal Vertically opposite angles are equal.
Check the fact really does give that answer
∠BPE=∠APQ=43∘ Using the fact on angle APQ gives 43°, which is exactly what was written down.
Check the numbers in the whole chain
43∘→43∘→43∘ Both steps give the numbers that were written down, so the working is right — only the reason for the second step is in question.
Rule out one of the other answers
Alternate angles are equal "Alternate angles are equal." is a true fact and it happens to give the same number, but it is the wrong reason — angle APQ and angle BPE are not that kind of pair, so it would earn no mark.
Rule out one of the other answers
Corresponding angles are equal "Corresponding angles are equal." is a true fact and it happens to give the same number, but it is the wrong reason — angle APQ and angle BPE are not that kind of pair, so it would earn no mark.
Rule out one of the other answers
Co-interior angles add up to 180∘ "Co-interior angles add up to 180°." is a true fact, but it does not describe this pair: applying it here would give 137°, not 43°.
Rule out one of the other answers
Vertically opposite angles add up to 180∘ "Vertically opposite angles add up to 180°." is simply not true, so it cannot justify anything. Used here it would give 137°.
Compare the two steps
step 1: Alternate angles are equal,step 2: Vertically opposite angles are equal The two steps use different facts. Naming the first one for the second step is the commonest error here.
Check the fact against the figure once more
Vertically opposite angles are equal Angle APQ and angle BPE are opposite each other at the point where two straight lines cross.
Write the reason out in full
Vertically opposite angles are equal A reason like "parallel lines" on its own earns nothing: the fact has to be named. Vertically opposite angles are equal.
Note the mark scheme
1 mark: correct fact named One mark is available and it is for naming the correct angle fact.
State the fact that earns the mark
Vertically opposite angles are equal The reason that must be written down is: Vertically opposite angles are equal.