The correct answer is option D: None of these.

Phaeophyceae, also known as brown algae, are a group of multicellular algae that belong to the kingdom Protista. They are mostly marine organisms and are commonly found in temperate and cold waters. Phaeophyceae exhibit various modes of reproduction, including vegetative, asexual, and sexual reproduction.

Let's break down the given options and understand why they are all correct except for option D:

a) Vegetative reproduction occurs by fragmentation: Vegetative reproduction is a type of asexual reproduction where new individuals are formed from vegetative parts of the parent organism. In the case of Phaeophyceae, fragmentation is a common method of vegetative reproduction. Fragments of the algal thallus break off and develop into new individuals. This process allows for the rapid colonization of new areas.

b) Asexual reproduction is by biflagellate pear-shaped zoospores: Asexual reproduction in Phaeophyceae occurs through the production of zoospores. Zoospores are pear-shaped cells equipped with two flagella, which enable them to move through water. These zoospores are released from specialized structures called sporangia and can swim to new areas before settling and developing into new individuals.

c) In sexual reproduction, gametes are pyriform and bear 2 laterally attached flagella: Sexual reproduction in Phaeophyceae involves the fusion of male and female gametes. The male gametes, called antherozoids, are pear-shaped cells with two laterally attached flagella. Similarly, the female gametes, called eggs or oospheres, are also pyriform and bear two laterally attached flagella. These flagellated gametes are released into the water, where fertilization occurs.

d) None of these: This statement is incorrect. As explained above, options a, b, and c are all correct statements about Phaeophyceae. The incorrect option suggests that none of the statements are correct, which is not true.

In conclusion, the only incorrect statement about Phaeophyceae is option D: None of these. Vegetative reproduction occurs by fragmentation, asexual reproduction is by biflagellate pear-shaped zoospores, and in sexual reproduction, gametes are pyriform and bear two laterally attached flagella.