zustimmend
approving
Expressing approval or agreement.
Als ich meinen Vorschlag beendet hatte, hörte ich ein zustimmendes Murmeln im Raum.
When I had finished my proposal, I heard an approving murmur in the room.

approving
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approving
Expressing approval or agreement.
Als ich meinen Vorschlag beendet hatte, hörte ich ein zustimmendes Murmeln im Raum.
When I had finished my proposal, I heard an approving murmur in the room.

'Zustimmend' is the present participle of the verb 'zustimmen' (to agree). This form is used like an adjective and describes an action or attitude that is happening at the same time.
Der Chef hörte sich meinen Vorschlag an und nickte zustimmend.
The boss listened to my proposal and nodded approvingly.
You can use 'zustimmend' in two ways: as an adverb that describes a verb, or as an adjective before a noun.
Sie lächelte mich zustimmend an.
She smiled at me approvingly.
Common combinations are 'zustimmend nicken' (to nod approvingly), 'zustimmend lächeln' (to smile approvingly), or as an adjective in 'eine zustimmende Antwort' (an approving answer) and 'ein zustimmendes Geräusch' (an approving sound).
Auf meine Frage erhielt ich nur ein zustimmendes Nicken.
In response to my question, I received only an approving nod.
agreed
'einverstanden' and 'zustimmend' both have to do with agreement. 'einverstanden' describes that a person really agrees to a plan or suggestion. 'zustimmend' is more about how someone shows agreement, for example by nodding, with a look, or in a tone of voice. That is why you naturally say 'Ich bin einverstanden' (I agree / I'm okay with that), but normally not 'Ich bin zustimmend'. Learners may confuse the words because both respond positively to a suggestion. As a state, 'einverstanden' fits; for behavior or expression, 'zustimmend' fits better.

Der Fahrer nickt zustimmend, als der Kunde die neue Route vorschlägt.
The driver nods approvingly when the customer suggests the new route.
Sie ist einverstanden, nach der Arbeit noch kurz bei ihrer Mutter vorbeizugehen.
She is okay with stopping by her mother's briefly after work.