Let's analyze each statement:

Statement A: Asexual reproduction occurs usually by biflagellate zoospores. - This statement is true for Phaeophyceae (brown algae). Asexual reproduction in these algae generally involves the release of biflagellate zoospores that swim away and eventually settle and grow into new individuals.

Statement B: Sexual reproduction is by oogamous method only. - This statement is partially true. While sexual reproduction in Phaeophyceae often is oogamous (involving a large non-motile egg and a smaller motile sperm), there are instances where isogamy (both gametes are motile and similar in size) occurs, though it is less common. Thus, the statement might be misleading as it implies that oogamy is the only method of sexual reproduction.

Statement C: Stored food is in the form of carbohydrates which is either mannitol or laminarin. - This statement is correct. In brown algae, the primary carbohydrates stored are mannitol and laminarin. These serve as crucial energy storage compounds.

Statement D: The major pigments found are chlorophyll a, c and carotenoids and xanthophyll. - This statement is true. The brown algae have chlorophyll a, c, various carotenoids, and xanthophylls like fucoxanthin which give them their distinctive brown color.

Statement E: Vegetative cells have a cellulosic wall, usually covered on the outside by a gelatinous coating of algin. - This statement is true. Brown algae have cell walls made of cellulose and are often covered by a layer of alginates, which can form a gelatinous coating. This helps in providing structural support and protection against dessication and mechanical damage.

Given these analyses:

• A, C, D, and E are true.
• B is not exclusively true as it suggests that oogamy is the only form of sexual reproduction.

Thus, the correct option is:

Option C: A, C, D and E only