Question

Alloys like manganin and constantan are used for making standard resistance coils as they have

Answer 1

Alloys are the combination of metals and other elements. Certain other metals/elements can be added to the metals in a specific ratio to form an alloy to confer certain properties to metals or strengthen some of their present properties.

Complete answer:
Constantan, also known as Eureka, is an alloy of copper and nickel, was discovered in $1887$ and has been used for resistance all along, apart from other applications. Its preference for resistance wire stems from high resistivity, low temperature coefficient of resistance, ability to attain good resistance values in small size , stability, ability to draw in thin wires. It’s melting point is also high at over $1200^\circ$ Celsius.
Manganin is an improvement over constantan, discovered in $1892$. It is an alloy of copper, manganese and nickel. Its temperature coefficient of resistance is near zero, and this makes it ideal for ammeter shunt, and high precision resistance.
It is used in resistance standards, its melting point is around $1000^\circ$ Celsius, and maximum usage temperature is limited to $300^\circ$ Celsius.
In other words, they have changed a lot less with temperature changes than other resistor materials. For very accurate circuits you don’t want a resistor that sets gain, amplitude, ratios and other factors to change with temperature.
Cheap resistors can change $5\% - 10\%$ over $- 20^\circ C$ to $80^\circ C$ or about $100^\circ$ degrees. They come with $5 - 10\%$ tolerance. Good metal film resistors see about $+ / - 100$ ppm per degree Celsius which is $1\%$ over $100^\circ$ degree change, these come with $1\%$ initial tolerance. They are used almost everywhere as they aren’t expensive for industrial use.

Note:
Constantan is an alloy of copper $(55\% )$ and nickel $(45\% )$ where manganin is an alloy of copper $(84\% )$ manganese $(12\% )$ and nickel $(4\% )$.
Really good resistors come as low as $5$ ppm per degree Celsius. That means a $.05\%$ change in resistance over $.05\%$ degree, although they will often be used over wider ranges. These often come with $.1\%$ or $.01\%$ initial tolerance.