A buoy is held inside water contained in a vessel by tying it with a thread to the base of the vessel. Tension in the thread acts in which direction?
A. Vertically up
B. Vertically down
C. horizontally
D. there is no tension in the string

Hint: Archimedes principle is the greatest invention which helps to solve the problems in fluid mechanics. It deals with the weight of the fluid displaced when an object is immersed inside the fluid and a new term called buoyancy force came into existence after that. Based on this concept only we solve this problem.

Complete answer:
Usually, when we immerse an object in the water then that object will displace some volume of the water and that will float on the water. That is floating in the sense it is in equilibrium which means forces are balanced. Weight will be always acting downwards but the object is not moving down it is floating which means there must be a force acting upwards which balances this weight and that force is called buoyancy force.
According to Archimedes principle the weight of the fluid displaced will be equal to the buoyancy force acting on the object.
Now the nature of the buoy is to float on the water with some volume outside the water. This clearly means that if we forcefully dip the entire buoy into water then buoyancy force becomes dominating and the buoy will try to move up to the previous state where weight will balance the buoyancy force.
In the question it is given that the buoy is completely dipped and tied with string to base. It's obvious that if they had not tied then buoy would have moved to the surface top float. So there is some extra downward force which adds up to the weight of the buoy and balances the buoyancy force and that force will be tension in the string. Hence tension acts vertically downward.

Hence option B would be the answer.

Note:
Objects whose density is lesser than the density of liquid only float on the liquids. If the density of an object is more then it will sink inside the liquid. The buoy is made in such a manner that it will float. Some birds float on the liquid by using this principle or we can tell that if the weight of the body is greater than the weight of the displaced liquid then the body will sink.