The cladode among the following is:

CONCEPT: A cladode is a photosynthetic stem branch of LIMITED growth, representing only one or two internodes, that takes over the leaf's function in a plant whose true leaves are reduced to scales or thorns. It is a specific, narrower case of the broader phylloclade -- a photosynthetic stem modification of UNLIMITED growth that may extend across many internodes and is usually flattened and fleshy.

APPLICATION: Among the four genera listed, the plant whose green branches are of one or two internodes, arising in the axil of a reduced scale leaf, is Asparagus. Its true leaves are reduced to minute scales/thorns, and photosynthesis is carried out entirely by these short, needle-like branches -- the textbook example of a cladode.

CROSS-CHECK: Setting the four options side by side by the origin of their defence/photosynthesis-related structure confirms the match:

• Parkinsonia -- a thorny xerophytic shrub whose spine develops from the tip of the leaf rachis -- a leaf-derived structure, not a photosynthetic stem modification like a cladode.

• Acacia -- a thorny xerophytic tree/shrub whose paired spines typically develop from the stipules at the leaf base -- again leaf-derived, not a photosynthetic stem segment.

• Opuntia -- a cactus whose fleshy, flattened stem segments span MANY internodes of unlimited growth -- the broader phylloclade category, not the narrower one-internode cladode.

• Asparagus -- the genus whose stem branches are limited to one or two internodes -- matching the cladode definition exactly.

Result: the plant with a true cladode among the four is Asparagus.