Find the True statement
a. Stress relaxation is time- and temperature-dependent
b. Polymers always have a melting temperature
c. Polyethylene becomes weaker if H is replaced with other radicals inside branches
d. Crazing indicates cracks inside the polymer

Option A is correct.

The only completely true statement in all of the available options is that "Stress relaxation is time- and temperature-dependent"

Explanation:

Stress relaxation is the constant decrease of stress with time in a material which is exposed to a constant strain at a constant temperature.

It is similar in concept to vaccination; in that, the material is being prepared to be able to undergo a particular amount of stress at a specific temperature in order to make the stress levels in the material lower than it usually should be.

It is evident that stress relaxation, (the extent), in a material completely depends on the amount of time that the constant strain is applied and the temperature at which this whole process occurs.

Polymers always have a melting temperature.

While it is true that almost all materials can melt. For Polymers, it is not necessarily true that the melting takes place at a constant temperature. In very complex Polymers, melting occurs over an appreciable range of temperature, not sharply as in some simple polymers.

Hence, polymers don't always have a melting temperature, some polymers melt over a range of temperature. So, this statement isn't completely true.

Polyethylene becomes weaker if H is replaced with other radicals inside branches.

This also is a statement that isn't always true. Replacing the H in some of the branches of polyethylene with radicals such as the carboxylic group, the hydroxyl group etc., have been shown to seriously strengthen the polyethylene. Polymers formed by replacing the H with ion-Like radicals are known as ionomers.

These groups have a polarity, hence, they are drawn together by their charges, come together in microdomains, toughening and strengthening the polyethylene without taking its ability to be cast to permanent shapes away.

This statement isn't always true too.

Crazing indicates cracks inside the polymer.

This is also false. Crazing doesn't indicate cracks in materials. What it does is that Crazing precedes the cracks.

Crazing is a phenomenon that entails the formation of microvoids in a material in response to excessive tensile stress being applied to the material. The microvoids (which forms in a plane normal to the tensile stress that caused them) often leads to cracks or fracture later.

So, Crazing doesn't indicate cracks, rather, it precedes them in most cases.

Hope this Helps!!!

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