A. The pitcher of Nepenthes is the modification of leaf lamina which takes the form of pitcher to trap insects.
B. Thorns of citrus are modified stem which performs the function of climbing or defense. Thorns are present in the axil of leaves or apex of stem.
C. A tendril of cucumber is modified stem which is formed by the modification of terminal bud into tendril because the stem is often weak in such plants.
Hence, thorns of citrus and tendrils of cucumber are stem modifications while pitcher of Nepenthes is leaf modification.
So, the correct answer is 'Pitcher of Nepenthes'.