This question already has an answer here:
Is it possible to rotate marker symbols (e.g. a triangle) in QGIS individually with an attribute field like in ArcGIS?
Edit: I would like to rotate each marker symbol individually in category A with an attribute field X, where the angle is defined.
This is how it is done in ArcGIS: "To rotate marker symbols individually based on the value of a field in the point layer's attribute table click Layer Properties dialog box > Symbology tab > Advanced > Rotation. On the Rotate dialog box, the Rotate Points by Angle in this field specifies the field that holds angular values and the Rotation Style controls the origin and direction of rotation" (ESRI Help).