Introduction:
When a material is subjected to an external force, it undergoes deformation. The rate at which this deformation occurs is known as the strain rate. Different mechanical properties of a material can be affected by the strain rate. In this context, we will discuss which property is more sensitive to an increase in strain rate.

Explanation:
To understand which property is more sensitive to an increase in strain rate, let's briefly define each of the properties mentioned in the options:

1. Yield strength: The yield strength is the point at which a material begins to deform plastically, i.e., permanent deformation occurs without any increase in the applied load.

2. Proportional limit: The proportional limit is the maximum stress a material can withstand without undergoing permanent deformation.

3. Elastic limit: The elastic limit is the maximum stress a material can withstand and still return to its original shape after the load is removed.

4. Tensile strength: The tensile strength is the maximum stress a material can withstand before it fails in a tensile test, i.e., the maximum stress a material can withstand while being pulled apart.

Sensitivity to Strain Rate:
Among the given options, yield strength is the property that is more sensitive to an increase in strain rate. Here's why:

1. Strain rate sensitivity: The strain rate sensitivity of a material determines its ability to deform at different rates. Some materials may exhibit a higher strain rate sensitivity, which means their mechanical properties can vary significantly with changes in strain rate.

2. Plastic deformation: Yield strength is a measure of a material's ability to resist plastic deformation. When the strain rate increases, the material experiences more rapid deformation, causing a higher strain rate at the onset of plastic deformation.

3. Strain rate hardening: Strain rate hardening refers to the increase in yield strength with an increase in strain rate. Some materials exhibit strain rate hardening behavior, where the yield strength increases as the strain rate increases.

4. Dynamic loading: In many real-world applications, materials are subjected to dynamic loading conditions, where the strain rate is relatively high. In such cases, the yield strength becomes a critical property in determining the material's response to the applied load.

Therefore, among the given options, yield strength is the property that is more sensitive to an increase in strain rate. Changes in strain rate can significantly affect the yield strength of a material, making it an important consideration in designing and analyzing materials for dynamic loading conditions.