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Shreyasi majumdar 1 Best Answer
correct option is b. Due to resonance C and N bond gets double bond character at r.t and rotation is restricted with makes the two set of methyl protons different, thus giving two singlets. As we increase the temperature, bond rotation becomes easier and hence at 150°, the two methyl groups become identical and give a singlet. https://youtu.be/vnUO0Myge90