Phellogen and Phellem respectively denote

Phellogen and Phellem respectively denote Cork cambium and cork.

Due to activity of vascular cambium the stem continue to increase girth-wise. So the outer cortical and epidermis layers get broken and are replaced by new protective cell layers.

So in cortex region another meristematic tissue develops called Cork Cambium or Phellogen.

Phellogen cuts off cells on both sides. The outer cells differentiate into cork or phellem.

Hence the correct answer is option (b).