Questions

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

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'.

Here the given compound is MgSO4.

In the given compound, Sulphur (S) and Oxygen (O) are two non metals and Magnesium (Mg) is a metal.

In this compound, Sulphur and Oxygen reacts to form covalent bond and the metal Magnesium reacts with both non metals Sulphur and Oxygen to form ionic bonds.

Hence, in the given compound MgSO4 there are both covalent and ionic bonds.