Daniel invited Nick to his house for a party, which turned out to be a sales pitch for the fitness equipment line that Daniel had recently started selling. Nick didn't need any fitness equipment, but since he was already at Daniel's house and enjoying the complimentary food and drink, he felt obligated to purchase something. Daniel is using the:

rule of reciprocity.

Explanation:

The rule of reciprocity refers to the social norm in which you feel obligated to do something for the person in response to what that person has done something for you. The concept of rule of reciprocation was developed by Robert Cialdini in his book titled "Influence: Psychology of Persuasion."

According to this principle, it is a universal habit in humans that one feels obligated to reciprocate to an act of generosity that has been offered by others either in a form of a material gift or a good deed. The rule of reciprocation is also used as a tool to persuade.

In the given case, Daniel has used the principle of reciprocity. Daniel invited Nick for a party which was nothing but a sales pitch. He offered him complimentary food and in response, Nick felt obligated to reciprocate buy a product from Daniel.

So, the correct answer is the rule of reciprocity.

uses : -

1.aneroid barometers primarily measure whether pressure is rising or falling.

2.aneroid barometers are often used for reading pressure in homes and recreational boats.  

3.other uses may include barographs in meteorology and altimeters in modern aircraft.  

4.this type of barometer can announce whether a high or low pressure system is coming or whether it is becoming more intense.  

5.there are different types of aneroid barometers, such as the altimeter, which measures altitude, or distance above sea level.  

6.another example is a barograph, which is a modified aneroid barometer that provides constant readings of atmospheric pressure on graph paper.  

7.when used in this capacity, the aneroid barometer can store a week's worth of data.

android diameter a device for measuring atmospheric pressure , often specially calibrated for use as an altimeter ,consisting of a box or chamber partially exhaused of air, having an elastic top and a pointer to indicate the degree of compression of the top caused by the external air

refer to the attachment for the characteristics : -

when your a dog and you need a bone.   go and eat a rabbit