The original sentence, 'One should keep his promises,' contains a common grammatical error related to pronoun agreement. The indefinite pronoun 'one' refers to people in general, and traditionally, it was followed by 'one's' or 'his/her' to maintain agreement. However, in modern English, using 'his' can be seen as gender-specific and exclusive. To ensure gender neutrality and inclusivity, the use of 'their' as a singular, gender-neutral pronoun is widely accepted and preferred, especially in informal and semi-formal contexts.
Therefore, 'One should keep their promises' is the most appropriate and grammatically sound correction. It maintains agreement while adopting contemporary usage that avoids gender bias. Options B, C, and D introduce other grammatical issues or incorrect pronoun choices for 'one'.