Betty Chan is at the mall buying things for her kids, which means she is shopping today. Option A is correct.
The question asks us to choose the word or words that best complete the sentence based on the context provided in the passage. According to Betty Chan's statement, she usually stays home on Sundays, but on this particular Sunday, she is at the mall doing something different.
Since she is at the mall, the correct answer is that Betty is shopping for her kids today. This is because the passage explicitly mentions that she is 'buying some things for my kids' which is synonymous with shopping.
Hence, A. is the correct option.