One strand of a short piece of DNA reads A T G C. What does the matching strand read?

Explanation

Each base is replaced by its own partner while the order is kept, so A gives T, T gives A, G gives C and C gives G. Copying the strand unchanged would put A opposite A, which the pairing rule never allows, and a version that partners the first two bases correctly but then sets G against G and C against C applies the rule to only half of the sequence.