Final answer:
A strap option (c) cannot be used effectively as a modifying prop in the Triangle pose, whereas a block, a wall, or a chair can offer support and facilitate proper alignment in the pose.
Step-by-step explanation:
The prop that cannot be used as a modifying prop in Triangle pose (Trikonasana) is c. a strap. The Triangle pose is a standing yoga pose that typically requires a straight line from the fingertips of one hand to those of the other, and the sides of the torso to be long and strong. Props such as a block, a wall, or a chair can be used to provide stability, and support, and to facilitate proper alignment.
However, a strap is generally used to extend the reach or to hold a limb in place, which is not as applicable in Triangle pose, where no limbs are being held by the hands and the emphasis is on sideways extension rather than lengthening outwards or binding.