Fill in the blank with a suitable preposition.
Sam was being careful _______ the spade, as it was sharp.
The preposition ‘with’ is used to denote a tool. 'With' is added after 'careful' to indicate that one needs to be cautious while handling a particular object or tool. Since the spade is sharp one needs to be careful while handling it to avoid injury. Hence option C is correct. The prepositions ‘on’ and ‘within’ are both used to denote a place/position and being inside something respectively. Since the noun here is a tool which is used for a purpose ‘on’ and ‘within’ are inappropriate. Hence options A and D are incorrect. On the other hand ‘for’ is used to show why something is done, i.e. the reason. If we use ‘for’ in the sentence it would mean that Sam is being careful 'for' the sake of the spade and not because he will be injured by it. Therefore option B is incorrect.