Refer to figure.
The geostrophic wind is a theoretical wind that results from the exact balance between the pressure gradient force (directed from high to low pressure, perpendicular to isobars) and the Coriolis force (deflecting moving air to the right in the northern hemisphere and to the left in the southern hemisphere). 

When these two forces are in perfect equilibrium the resulting wind flows parallel to straight isobars, with low pressure on the left in the northern hemisphere (Buys Ballot's Law). The geostrophic wind exists at altitudes above the friction layer (above approximately 2000 ft AGL) where surface friction is negligible, allowing the two-force balance to be maintained. In areas with straight isobars the upper wind approximates the geostrophic wind closely. 

Where isobars are curved (cyclonic or anticyclonic flow), the wind is called the gradient wind, which adds a centrifugal force component to the balance. 

Which flows parallel to straight isobars → CORRECT. The geostrophic wind is the result of the balance between pressure gradient force and Coriolis force and flows parallel to straight isobars in the frictionless upper atmosphere. This is the standard definition of the geostrophic wind and is the correct answer.

Which flows perpendicular to the isobars from high to low pressure → INCORRECT. Wind flowing perpendicular to isobars from high to low pressure describes the pressure gradient force direction, not the wind itself. In reality the Coriolis force deflects the wind until it flows parallel to the isobars. Perpendicular flow would only occur if there were no Coriolis force (near the equator where Coriolis approaches zero).

Which flows parallel to curved isobars in a cyclonic pattern → INCORRECT. Wind flowing parallel to curved isobars is the gradient wind, not the geostrophic wind. The gradient wind accounts for the centrifugal force associated with curved flow in addition to the pressure gradient and Coriolis forces. The geostrophic approximation applies specifically to straight isobars.

Which is significantly affected by surface friction below 2000 ft → INCORRECT. The surface wind below approximately 2000 ft is affected by surface friction, which reduces wind speed and causes it to cross isobars toward low pressure. This describes the boundary layer wind, not the geostrophic wind. The geostrophic wind exists in the frictionless upper atmosphere above 2000 ft.