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Solution: a)

Reasoning:

Arguments against strukture b):
  • COOH-signal missing,
  • CH2 -doublet missing,
  • -CH= should be a multiplet.
Arguments for structure a):
  • 5 signals,
  • -OCH3 d= 3,6 ppm (Singulett)
    -CH=CH- d = 6,3 ppm (Dublett)
    d =7,2 ppm (Dublett)
    Aromat d = 7,1 ppm (Dublett)
    d = 8,3 ppm (Dublett)
This was an example to make an decision using the existence or the lack of coupled multiplets.

On to the next problem!