Do I use the singular or plural form – letter/letters?

Question

Do I use the singular or plural form – letter/letters?

1. In the paragraph if there is a cross above the letter A or B, highlight them blue.
2. In the paragraph if there is a cross above the letters A or B, highlight them blue.

There are many A’s and B’s in the paragraph and I’m not sure if I need to use the plural. If I use the singular should I say ‘highlight them blue’ or ‘highlight it blue’?

Answer 1

Given that there is an implied parallel construction in the sentence using A and B with the conjunction or, the expansion might be

if there is a cross above the letter A or ( the letter ) B

this is in contrast to

if there is a cross above any of the letters A, B, or C

The number of occurrences of the letters does not matter when describing the designation of which letters to use.

The rest of the sentence refers to the cross above the designated letters and should be singular to reflect same noun-verb plurality

... is a cross ... highlight it ( in ) blue.

Answer 2

In the paragraph, if there is a cross above a letter A or B, highlight it blue.

You would use the indefinite article a since there are many of each letter. You would keep letter and it singular since the action is taken individually (varies based on the condition of each letter).

This might read more naturally as:

In the paragraph, if there is a cross above any letter A or B, highlight it blue.