Read the vignette as a sorting problem between four near neighbours. The hard facts are: bodily over-arousal (racing heart, broken sleep, reduced appetite), some low mood, but no hopelessness, no suicidal thinking, no triggering life event, and a woman still functioning fully at work and socially.
Knock out depression first. A genuine depressive episode is anchored by sustained low mood with hopelessness, anhedonia or suicidal ideas and typically dents functioning; she has none of these markers and is functioning normally, so $mild\ depressive\ episode$ does not hold.
Knock out adjustment disorder next. It demands a named stressor within roughly three months of onset, and the stem explicitly denies any precipitating event, which removes it.
Mixed anxiety and depression is a category for blurred, subthreshold cases where neither side leads. Here one side clearly leads: the autonomic, arousal-heavy complaints. That points to generalised anxiety disorder, in which pervasive worry shows itself through palpitations, insomnia and appetite change, with mood symptoms riding along secondarily.
The dominant, free-floating somatic anxiety with preserved function names the disorder.
\[\boxed{Generalised\ anxiety\ disorder\ (GAD)}\]