A.

presence or absence of a well-defined nucleus

RH Whittaker's classification is nor based on presence or absence of a well - defined nucleus. He gave five kingdom classification and used five criteria for delimiting kingdoms.
These are
both come under body organisation
(i) Complexity of cell structure
(ii) Complexity of body structure
(iii) Mode of nutrition
(iv) Ecological life cycle including mode of reproduction
(v)Phylogenetic relationship.
He divided organisms into five kingdoms as follows:
Kingdom  - Monera
Kingdom - Protista
Kingdom - Fungi
Kingdom - Plantae
Kingdom - Animalia

A.

cell membrane structure

Archeabacteria different from eubacteria in that eubacteria has two cell membrane composed mainly of glycerol - ester lipids, while archaebacteria have a membrane made up of glycerol - ether lipid. Ether lipids are chemically more resistant then ester lipids. This stability help archaebacteria survive at high temperature and in very acidic or alkaline environment. 

D.

Sphagnum

Peat is mainly an accumulation of partially decayed vegetation or organic matter and Sphagnum accumulations can store water since both living and dead plants can hold large quantities of water and living matter (like meat eggs) for long distance transport inside their cells hence, it is responsible for peat formation.

B.

being imperforate

Tracheids and vessels both are called tracheary elements because their main function is conduction of sap. They differ from each other in being inspectorate. Tracheids are the specific cells which pit to support upwards and lateral conduction of water sap. Tracheid are comparatively short and single cell, while vessels have more than one cell and up to 10 cm long.

D.

Mesosomes