Thermal equilibrium is reached electrical forces are balanced the sound vibrations reach a high pitch two objects in contact with each other are the same temperature objects have enough kinetic energy for them be at terminal velocity the net force on an object is zero

Option (c) is the correct answer.

Explanation:

When there is no exchange of heat between two substances which are physically connected to each other then it is said that thermal equilibrium has reached.

Also at thermal equilibrium the temperature for two objects or substance becomes same.  

Thus, we can conclude that out of the given options thermal equilibrium is reached when two objects in contact with each other are the same temperature.

no. dot product   =0 when vecs at right angles