Line from the tempest is written in iambic pentameter

B. "With raven's feather from unwholesome fen"

In order to qualify as being written in iambic pentameter, the line needs to first have 10 syllables.

With | ra | ven's | fea | ther | from | un | whole | some | fen
  1       2      3         4      5        6      7       8           9      10

Then, you need to check meter for stressed and unstressed syllables. To make this easier, look at two syllable words first. For instance, 'raven's' and 'feather' both put the emphasis on the first syllable. 'unwholesome' puts the emphasis on 'whole'. You can mark the line from there with alternating stress (/) and unstress (u), seeing if the rhythm fits. 

   u      /       u         /      u         /       u        /          u        /
With | ra | ven's | fea | ther | from | un | whole | some | fen