Here's my take on this question, which is a good one:

The key is understanding which base *should* be incorporated and going from there. The table headings mention a "G template" and an "oxo-G template." If a template strand of DNA includes guanine (or a base that is very structurally similar), then the base that should be incorporated into the new strand is cytosine. We can then conclude that C represents the correct insertions, while all other nucleotides (A, T, G) could only be incorporated in error.

From there, we need to understand how they chose to represent the numbers. The table mentions "log10 insertion frequency," so they've simply taken the actual insertion frequency and converted it to logarithmic form. In other words, if this table were NOT logarithmic, the "oxo-G" column could have read A: 10^-2, C: 10, T: 10^-2.1, G: 10^-4. That would mean that, for every 10 times that the template includes an oxo-G and C is correctly inserted, A will be INcorrectly inserted 10^-2 times. We can adjust these numbers however we want - for every 100 times that C is correctly included, A will be incorrectly included 10^-1 times; for every 1000 correct Cs, we'll observe 10^0 (or one) incorrect As, etc.

(Note: this is the kind of table that, due to its unusual nature and use of logarithms, 99% of students won't understand right away, and that's fine. When you aren't sure how to read a table, look for any unusual results - any values that are much larger than the others, etc. From the table, we see that the values for C are the only positive ones, so we must make sense of that somehow. From there, when we try to understand *why* C is positive, we're drawn to the "G template" and "oxo-G template" headings, and we can conclude that C is unique in that it is supposed to pair with G.)