Hint:Succulent plants are also called succulents. The word, succulent derived from the Latin word "succus" which means sap or juice. There are six different plant families that contain plant succulents Aizoaceae, Cactaceae, Crassulaceae have more succulents.
Complete answer: Succulents have thick, fleshy parts usually to store water in desert climates (or) dry conditions which makes them equipped to survive in an ecosystem. They possess a characteristic known as succulence. In addition to succulence, they also have water-saving features like CAM (crassulacean acid metabolism) to reduce water loss, reduction in the number of stomata, photosynthesis takes place in the stem rather than leaves. Ability to keep moisture in the roots even in high temperatures. They can be propagated by different methods. The most common way is vegetative propagation. They can be propagated from leaves, stem cuttings, pups or offsets, and also from seeds. In horticultural use, the word succulent usually excludes cacti. However, in botanical terminology, cacti are succulents but not all succulents are referred to as cacti.
Making a note of the options which have given in the question - A. Pisum belongs to the family Fabaceae which have seed-pods, B. Casuarina are deciduous trees that grow in open, coastal habitats, etc. C. Oxalis is a genus flowering plants in the wood sorrels and coming to the last option D. Euphorbia, it is a subtropical succulent species of the genus - Euphorbia. They store water in the stems same as succulents.
Hence, from the above discussion we can conclude that option-D is the right answer.
Note: Euphorbias have always been a close preference with succulent growers. These are one of the popular of all genera in succulent selections. Some species of Euphorbia are diecious, which means a single plant will only produce male or female flowers. Some of the best succulent Euphorbia species are Euphorbia milii, Euphorbia ingens, Euphorbia trigona etc.