Questions

The logic in the given puzzle is 

Addition of numeric values of corresponding letters in the first three figures gives respective corresponding letter in fourth figure. 

i.e, A = 1 , W = 23 , B = 2 => 2 + 1 + 23 =26 = 'Z'

Similarly, L = 12, K = 11 , A = 1 => 1 + 11 + 12 = 24 = 'X'

Here the missing letter is 'X'.

A Dome-shaped, muscular and membranous structure that separates as a partition of the thoracic and abdominal cavities is Diaphragm. 

Thoracic or chest cavity ::  It is the cavity where the heart and lungs are present. It is the second largest hallow space in the body.It also has vertebral column, ribs, breast bone,...

Abdominal cavity :: It is the cavity where the digestive tract, liver, kidneys, the spleen, pancreas,... are located. It is the largest hallow space of the body.

Australia is the smallest of all the continents