The original Roman alphabet lacked the following letters EXCEPT:

The story behind the answer

X is correct because the original Roman (Classical Latin) alphabet already included X, used to represent the /ks/ sound—and famously appears as the numeral for 10. By contrast, J, U, and W did not exist in early Latin: I served for both the vowel i and the consonant j sound, and V covered both the vowel u and the consonant v. W (“double V”) was a much later medieval innovation for Germanic languages.

A handy memory trick: think of Julius Caesar written the Roman way—IVLIVS instead of Julius—showing how I/J and U/V weren’t separate letters yet. And remember that Romans carved X on monuments and used it in numerals, proving X was part of their toolkit from the start.