Answer:
A, C, F
Step-by-step explanation:
Definition: The circumcenter is the center of the triangle's circumcircle - the circle that passes through all three of the triangle's vertices. If point H is the circumcenter of the triangle DEF, then the circumcircle passes through its vertices D, E and F (option A is true).
Option B is false, the circumcircle doesn't pass through the points L, M and N. This option is true for inscribed circle, not for circumcircle.
Option C is true, because HD and HE are the radii of the circumcircle.
Option D is false. This option is true for inscribed circle, not for circumcircle.
Option E is false. This option is true for inscribed circle, not for circumcircle.
Option F is true, because both these angles are right angles.