Final answer:
A bucket of sea water containing beach sand is a heterogeneous mixture, as the water and sand do not mix completely.
Step-by-step explanation:
A heterogeneous mixture is a mixture that is not uniform throughout, meaning that its components are not evenly distributed. Out of the options given, (A) a bucket of sea water containing beach sand is the only example of a heterogeneous mixture. In this mixture, the sea water and beach sand do not mix completely, and you can see distinct particles of sand within the water.