The problem is tedious to tackle.

Answer: Option C

Easy : achieved without great effort; presenting few difficulties.
Difficult : needing much effort or skill to accomplish, deal with, or understand.
Tiring : causing one to need rest or sleep; fatiguing.
Troublesome : causing difficulty or annoyance.
Tedious : too long, slow, or dull; tiresome or monotonous.

Synonym of Tedious is Tiring