reverso gives an etre and an avoir form for grossir. As this is intransitive in the sense that he has put on weight himself, I assumed it was the etre form? You marked it wrong.
The verb 'grossir' takes the auxiliary 'avoir' which seems odd as it is indeed intransitive in the case of putting on weight.
Just to recap , and according to 'Le petit Grevisse', the verbs that take 'avoir' are :
1. verbs avoir and être
2. all transitive verbs - acheter, prendre , copier etc.
3. most intransitive verbs - courir, parler, trembler, etc.
( not the verbs known as verbs of action - sortir, aller etc.)
4. all impersonal verbs
"Le Grevisse de l'étudiant" also describes verbs which, according to a traditional rule, take avoir when describing the action, but être when describing the result. "Grandir" is mentioned as an example, but "grossir" would also fit into this category. However it goes on to say that, in fact, the majority of these verbs only really take être when the past participle is employed as an adjective.
"Le Bon Usage" includes "grossir" as an example of this type of verb, but adds that the rule is more theoretical than practical.
But this does explain why some verb tables will give être as an alternative.
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