The correct option is C watching movies
The comparison is between two activities 'building sandcastles' and 'watching movies'. The act of 'watching' is missing from the sentence, though we can figure out the action through its context, still it's grammatically incorrect and needs to be changed. Option A is wrong and since we're correcting the underlined word by adding 'watching before it' option B is correct. Options C and D both are incorrect as they don't include the watching and are also grammatically incorrect.