Hint: Plants consist of an intact and impenetrable barrier which consists of a waxy cuticle and a bark which is the first line of defence. They protect themselves by emitting volatiles which attracts the enemy of the insect. Some plants produce antimicrobial chemicals which helps them to fight against the pathogen consisting of other antimicrobial proteins and antimicrobial enzymes.
Plant prey defence includes sharp spines, tough leathery leaves, and pointed leaves. The plant consists of impenetrable barriers to protect them from herbivores. When the herbivore reaches the plant barrier it responds with secondary metabolites that are often toxic compounds that consist of glycol cyanide that harms the herbivores. It promotes the release of volatile compounds on attacking by the Predator which damages the plant tissues and releases Jasmonate hormones. These volatile compounds attract the parasitoids and kill the predators as host insects. Other defence includes adaptation against herbivores such as hard shells, spines which are modified leaves, and the thorns which are modified branches. They discourage the animals and cause damage which includes rashes and allergic reactions. The thorn-like stipules of the Acacia collins offer shelter to the ants and in return get protected against herbivores. The spines of the cactus plant act as a mechanical defence against predators.
Additional information: The exterior protection of the plant is compromised by the mechanical damage which provides an entry point for the pathogens. The plant must set different defence mechanisms such as toxins and enzymes if the first line of defence gets breached. Some metabolites which are compounds that are not derived from photosynthesis and are not necessary for the process of respiration and plant growth and development, are toxic and lethal to animals that ingest them.
So, the correct answer is '(d) All of these'.
Note: Some plants containing certain defence structures suggest the presence of prickles on the stems of Rose, thorns which are present on the branches with the sharp tip and the spines which are found in Cactus and Ocotillo whose leaves are dried.