The correct answer is option 4. Manganin and constantan alloys are used in wire-bound standard resistors because they have resistivity that remains relatively unchanged with temperature.

Resistivity refers to the inherent resistance of a material to the flow of electric current. In the case of wire-bound standard resistors, it is important for the resistivity of the chosen material to remain stable over a wide range of temperatures to ensure accurate and consistent resistance values.

Option 1, which states that these alloys are cheap and easily available, is not necessarily the reason why they are used. While cost and availability may be factors to consider, the primary reason for selecting manganin or constantan is their temperature stability.

Option 2, which claims that these alloys have high resistivity, is incorrect. Manganin and constantan actually have relatively low resistivity compared to other materials.

Option 3, which suggests that these alloys have low resistivity, is also incorrect. As mentioned earlier, manganin and constantan have low resistivity, but this is not the primary reason for their selection in wire-bound standard resistors.

In summary, the main advantage of using manganin or constantan alloys in wire-bound standard resistors is their ability to maintain