Determine the value of ab and c with reasons.

Angle $a$ and the $85^\circ$ angle are vertically opposite angles. Vertically opposite angles are equal.

$$a = 85^\circ$$

Reason: Vertically opposite angles.

Angle $b$ and the $85^\circ$ angle form a linear pair on the straight line HI. Angles in a linear pair sum to $180^\circ$.

$$b + 85^\circ = 180^\circ$$ $$b = 180^\circ - 85^\circ$$ $$b = 95^\circ$$

Reason: Angles on a straight line (linear pair).

Angle $c$ and angle $b$ are alternate interior angles. Since lines CD and EF are parallel, alternate interior angles are equal.

$$c = b$$

Since $b = 95^\circ$:

$$c = 95^\circ$$

Reason: Alternate interior angles are equal.

Angle $d$ and the $85^\circ$ angle are corresponding angles. Since lines CD and EF are parallel, corresponding angles are equal.

$$d = 85^\circ$$

Reason: Corresponding angles are equal.

The values are:
$a = \boxed{\text{85}^\circ}$
$b = \boxed{\text{95}^\circ}$
$c = \boxed{\text{95}^\circ}$
$d = \boxed{\text{85}^\circ}$