Question: 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).

Answer: 
Use **Data defined override**. Go to *Layer Properties* and in *Simple marker* style open the Data defined override menu with icon next to the *Angle* value. Choose `Field Type -> "your column"`.

Data defined override button:
[![enter image description here](https://i.stack.imgur.com/3L0S2.png)](https://i.stack.imgur.com/3L0S2.png)

Select column:
[![enter image description here](https://i.stack.imgur.com/feb7K.png)](https://i.stack.imgur.com/feb7K.png)

Result:

[![enter image description here](https://i.stack.imgur.com/sKc3i.png)](https://i.stack.imgur.com/sKc3i.png)

If you select `Edit...` in `Data defined override` you can also insert a custom expression.
If u want rotate symbol for a specific Category go style -> change symbol method to Rule-Based ->add new rule like "column" = 'something' and chose rotation angle

Edit: solution to your problem: click icon next to rotation input field -> click edit (under "Variable") -> type "column"

Where "column" is the name of column where angles are defined