"Grandfather cannot read the letters that I write."

Translation: Kakek tak bisa membaca huruf yang aku tulis.

[donny309620]

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.

[ChrisNeedh5]

I agree with donny, it is unclear which type of 'letters' this refers to.

[JimCraney]

This is very ambiguous. "letters" in this case meaning alphabet, not as in post. In this case, both answers should be marked correctas a face-to face teacher would mark.