Introduction:
Differential scanning calorimetry (DSC) is a widely used technique in the field of chemistry to measure the thermal behavior of materials. It involves the measurement of the heat flow into or out of a sample as it is subjected to a controlled temperature program. DSC can provide valuable information about various thermal transitions, including glass transition temperatures, softening points of amorphous polymers and glasses, and melting points of crystalline polymers.

Glass Transition Temperatures:
Glass transition refers to the transition from a hard and brittle state to a soft and viscous state in amorphous materials, such as polymers and glasses. During this transition, there is a change in the molecular arrangement and mobility of the material. DSC is capable of detecting this transition by measuring the change in heat flow when the material is heated or cooled. The glass transition temperature (Tg) is the temperature at which this transition occurs. DSC can accurately determine the Tg of a material by analyzing the corresponding heat flow data.

Softening Points of Amorphous Polymers and Glasses:
The softening point of a material is the temperature at which it starts to deform under a specified load or pressure. In the case of amorphous polymers and glasses, DSC can be used to determine the softening point by monitoring the heat flow as the material is heated. As the temperature increases, the amorphous material gradually becomes softer and eventually starts to deform. This softening can be observed as a change in the heat flow measured by the DSC instrument.

Melting Point of a Crystalline Polymer:
Crystalline polymers have a well-defined melting point, which is the temperature at which the crystalline regions of the polymer structure start to melt and transition into a more disordered state. DSC can accurately determine the melting point of a crystalline polymer by measuring the heat flow during heating. As the temperature increases, the crystalline regions gradually melt, resulting in a characteristic endothermic peak in the DSC thermogram. The peak temperature corresponds to the melting point of the polymer.

Conclusion:
In conclusion, differential scanning calorimetry (DSC) is a highly useful technique for determining various thermal properties of materials. It can accurately determine the glass transition temperature (Tg) of amorphous materials, such as polymers and glasses. Additionally, DSC can be used to determine the softening point of amorphous polymers and glasses, as well as the melting point of crystalline polymers. Therefore, the correct answer to the given question is option 'A' - DSC is useful in determining glass transition temperatures.