Step 1: Understand the meaning of "TEDIOUS".
The word \textit{tedious} refers to something that is too long, slow, or dull—essentially, something that causes boredom or fatigue.
Step 2: Analyze the options.
PAINFUL: Involves physical or emotional pain, not necessarily boredom.
TIRESOME: Something that is boring or exhausting — matches perfectly.
TROUBLESOME: Refers to causing difficulty or annoyance.
LENGTHY: Refers to being long in duration but not necessarily boring.