Which one of the following is categorised as a parasite in the true sense?

$Cuscuta$ is an example of

In an aquarium,two herbivorous species of fish are living together and feeding on phytoplanktons. As per the Gause's Principle,one of the species is to be eliminated in due course of time,but both are surviving well in the aquarium. Give possible reasons.

In a humid climate,the presence of spines in shrubs is primarily:

Which of the following plants develops various structures for protection against herbivores?

Consider the following four statements and select the option which includes all the correct ones $:-$
$(a)$ Sparrow eating any seed is no less a predator.
$(b)$ Monarch butterfly is highly distasteful to its predator.
$(c)$ Nearly $25 \%$ of all insects are known to be phytophagous.
$(d)$ Thorns are the most common morphological defence mechanism.
Options $:-$