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Hi there,

Trans double bonds are highly unstable in small rings. They can be stable in larger rings such as an 8-membered ring.

(d) has a trans double bond in a 5-membered ring, that's why it is unstable.

I hope it helps. Let us know if you have more questions.

Hey there! Thanks for pointing this out! We'll fix this as soon as we can.

Hey there! For organic chemistry reactions, we only generally write the major or desired products. In this case, this is a substitution reaction, so we only wrote the substitution product. If we include the other products, the $Na^{+}$ from $CH_3ONa$ will combine with $Br^{-}$ from $CH_3Br$ to form NaBr. I hope this clears things up!