Final answer:
To form the word 'xes' by replacing the blank space with 'x', we need to use the letter 'x' itself.
Step-by-step explanation:
In this question, the word 'ces' is most likely a typographical error. Assuming the correct word is 'see', the question is asking to replace the blank space with 'x' to form the word 'xes'.
The correct answer is 'x'. When we replace the blank space with 'x', the word becomes 'xes', as required.