In Prasarita Padottanasana C the hands are clasped behind the back.
You then bend forwards while standing with legs apart, knees straight and feet parallel.
This same arm position is used in plow pose and can also be used in bridge pose.
If you have trouble grabbing your hands behind your back then you might find it helpful to practice this arm action in isolation while either standing or sitting.
To make grabbing your hands behind your back easier, you can first practice moving your shoulders up and back as in this shoulder exercise for the rhomboids.
With shoulder blades retracted you can then grab your hands behind your back. Once your hands are clasped don't need to focus so much on retracting the inner edges of your shoulder blades. Instead you can focus on pulling your shoulders back.
If you have tight shoulders and you can't grab your hands behind your back, an option is to grab a towel behind your back. You can then work your hands closer together using the towel or strap for traction.
Another option is to practice stretching the fronts of the shoulders. You can practice the shoulder stretch "lapasana" both with the palm facing upwards (slightly easier and shown below) and then with the palm facing downwards (so that the elbow points up.)
Other standing stretches that may help can be found in yoga shoulder stretches.
When you can clasp your hands you can play with rotating your arms either internally or externally. I'd suggest that you focus on opening the top of your chest if you rotate your arms externally. Bend your thoracic spine backwards. Open the thumb side of your palms with this rotation or open your palms completely and turn them outwards. (This does make it more difficult to continue to clasp your hands. You may have to engage your rhomboids to make this easier.)
Note that your elbows will be pointing inwards.
If you are internally rotating your arms then you may find it helpful to bend your thoracic spine forwards. Use your abs in this position and also see if you can activate pectoralis minor. You may find that the pinky side of your hands want to open in this variation and here again you can turn the palms backwards if you choose.
In this variation of the prasarita padottanasana c arm position your elbows will point out.
Personally I find that this latter option, rotating the arms inwards, can make it easier to get the hands to the floor when doing prasarita padotanasana c. The key is learning to use the pectoralis minor muscle to help pull the top of the shoulder blade (focus on the coracoid process, the bony peak at the top of each shoulder) forwards and down.