How to spell TREESIN correctly?

If you meant "trees in", then you can use those two words instead. If you meant to say "treason", then that's a significantly different word with a different meaning. In any case, always double-check your spelling to avoid confusion.

• resin The resin was poured into the mold and left to cure for a few hours.
• treason
• Treeing He was treeing a deer in his yard.
• Trees The trees were swaying in the wind as I walked through the forest.

Contempories

• contemporaneous
• contemporaries
• contemporary
• contemptuous
• cotemporaries