Sometimes we went off the road and on a path through the pine forest. The floor of the forest was soft to walk on; the frost did not happen it as it did the road. But we did not mind the hardness of the road because we had nails in the soles and heels nails bit on the frozen ruts and with nailed boots it was good walking on the road and invigorating. It was lovely walking in the woods... We found great joy on account of

The passage describes how nailed boots made walking on frozen roads enjoyable - the nails 'bit on the frozen ruts' making walking 'good and invigorating'. The sentence 'We found great joy on account of' is incomplete in the given text, but context clearly indicates the joy came from walking on frost with the security and grip provided by nailed boots. Other options (just wearing boots, the long walk, walking through forest) are not the direct source of joy mentioned.