The correct option is D Couldn't / can
"Could/can" in option A is incorrect as 'could' suggests the ability to ski. But, the use of 'but' suggests contrast. Thus, the first blank should indicate inability.
"Can/could" in option B is incorrect as both suggest ability. This is incorrect
"Could/can't" in option C is incorrect as the context does not agree with the ability to do something and the inability to do it later.
"Couldn't/ can" in option D is correct as the conjunction 'but' indicates a contrasting statement in the subordinate clause. Here, the subordinate clause also mentions 'very well', which suggests an expertness in the ability to complete an action. Thus, both options match the context.
Thus, option D is the correct answer.