"Grandfather cannot read the letters that I write."
Translation:Kakek tak bisa membaca huruf yang aku tulis.
I think it would help that if there are more than one options for a word to choose from (in this case "letters") that the correct answer should reflect the appropriate options. In this case: "Kakek tak bisa membaca huruf/surat-surat yang aku tulis". Not sure if a 'slah' (/) is the best way to present it - I will leave that to the experts.