Step 1: Examine Option A - "Grateful''
"Grateful'' is correctly spelled. It means feeling or showing appreciation.
Step 2: Examine Option B - "Rhythm''
"Rhythm'' is correctly spelled, though tricky, it has no vowels after the "h''.
Step 3: Examine Option C - "Liaison''
"Liaison'' is correctly spelled. It means a link or communication between groups.
Step 4: Examine Option D - "Acquaintence''
"Acquaintence'' is incorrect. The correct spelling is "acquaintance''. The ending should be "-ance'', not "-ence''.
Thus, the misspelled word is Option D: "Acquaintence'' $\rightarrow$ "Acquaintance''.