The answer of this question is c .
About "off the coast "
off the coast
1.离开海岸
2.离开海岸；在远离沿岸海面
3.在海面上
4.在海岸附近
off the coast of
1.离……海岸不远的地方
off the coast colombia
1.在哥伦比亚的海岸线附近
lie off the coast
1.离开海岸线
the boat sank off the coast
1.这只船在离海岸不远处沉没了
2.这只船在离海岸不远处沉没了
And about "on the coast"
on the coast
1.在海边
2.在海岸线上
3.在海岸上
4.在岸上,沿岸
sunset on the coast
1.落日
lie on the coast
1.在海岸线上
the town lies on the coast
1.该城位于海边
a town on the coast
1.有铁路通达的城镇
2.美国北卡罗来纳州大西洋海岸的一个海角
From the mean of this sentence we can know the answer is C.