Choose the letters whose water images are identical to the letter itself.

(A) k

(B) z

(C) x

(D) o

Choose the correct answer from the options given below:

A water image is a reflection across a horizontal axis, similar to viewing a letter's reflection in water.

Analyzing each letter:

(A) Letter 'k': When reflected across a horizontal axis, the vertical line remains in place, but the diagonal strokes flip to opposite sides. The water image does NOT match the original letter.

(B) Letter 'z': When reflected horizontally, the diagonal line flips orientation. The water image does NOT match the original letter.

(C) Letter 'x': When reflected across a horizontal axis, both diagonal lines swap positions symmetrically. Due to its symmetrical structure, the water image is identical to the original letter.

(D) Letter 'o': Being circular, it has complete symmetry in all directions. The water image is identical to the original letter.

Letters with water images identical to themselves: (C) x and (D) o

Therefore, the correct answer is Option 4: (C) and (D) only.