'Barnacles' growing on the back of whale is a classical example of:

Commensalism

The correct answer is Option (4) → Commensalism

Commensalism refers to an interaction between species where one species benefits while the other is neither harmed nor benefited.
For instance, an orchid growing as an epiphyte on a mango branch and barnacles growing on the back of a whale both derive benefits, while the mango tree and the whale do not appear to be affected positively or negatively.

Another example of commensalism is observed in the relationship between cattle egrets and grazing cattle. In farmed rural areas, these birds can often be seen foraging near grazing cattle. The egrets take advantage of the insects stirred up and flushed out by the cattle as they move, making it easier for the birds to find and capture their prey.

The cattle, however, are not affected by the presence of the egrets. Similarly, the interaction between sea anemones and clownfish demonstrates commensalism. The clownfish find protection among the anemone's stinging tentacles, which deter potential predators. The anemone does not seem to derive any apparent benefit from hosting the clownfish.