It is necessary to select appropriate experimental materials, reagents and instruments to identify some components in biological tissues and cells and observe their structures. The following materials and instruments (reagents A, watermelon juice has color, which will interfere with the observation of the color of the experimental results. A is wrong;

B, separating the photosynthetic pigment with chromatographic liquid, and extracting the photosynthetic pigment with anhydrous ethanol. B is wrong;

C, when observing the distribution of DNA in cells, the experimental material is oral epithelial cells, the reagent is methyl green, and the mitochondrial staining is janus green, C error;

D, high-power microscope observation chloroplast can choose fresh spinach leaves, tear off epidermal cells, mesophyll cells are less, D is correct.

Therefore, choose: d.