a. What is seen in the picture above?

Answer
Elschnig's pearls
These appear as globular opacities behind the implant. 
With retroillumination, the pearls appears as transparent masses.
 

b. What is responsible for this appearance?

Answer
The pearls are formed from aberrant attempt of remaining lens cells
attached to the capsule to form lens fibres. Microscopically, they appear as large clear lens cells (bladder or Wehl cells)
 

c. Are Elschnig's pearls the most common cause of decreased vision following cataract extraction? If not, what is the most common cause?

Answer
No. Elschnig's pearls usually involve the peripheral capsular and seldom 
affect the vision. 
Posterior capsular opacification is a more common cause of capsular opacity.
 

d. What is the responsible for posterior capsular opacification?

Answer
Posterior capsular opacification is caused by proliferation of the remaining 
epithelium. The proliferation results in multiple layers of epithelium over the posterior capsule that cause capsular opacification. Another important cause
for opacification is myelofibroblastic and fibroblastic differentiation of the epithelium giving rise to capsular contraction and wrinkles.