Bonjour
1- Tous les samedis matin, ils choisissent le parc.
a- Tous les samedis matin, is correct. "matin" has to stay singular. Used this way, it's an adverb so it doesn't agree.
BUT the other half of the sentence =>. "ils ont choisi le parc" is incorrect
b- ils choisissaient le parc. is correct
Just check, if matin is singular for the b)..; it's the best answer
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2- Nous mangeons en ville le samedi.
b- Nous allons manger en ville le samedi.
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3- Il va me téléphoner ce soir ?
b- Oui, il va te téléphoner.
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sorry for the number 1, it's confusing... if you have any question, please ask in comment..
hope this helps ☺☺☺