Power Transistor Characteristics:
A power transistor is a type of transistor that is designed to handle high power levels and currents. It is commonly used in applications such as power amplifiers, motor control circuits, and switching circuits. Here, we will discuss the given characteristics of a power transistor and explain why option B (1, 2, and 5) is the correct answer.

1. Cannot be easily paralleled:
Power transistors have a limited ability to be paralleled due to their inherent characteristics. When transistors are paralleled, each transistor shares the load current. However, due to manufacturing variations and thermal effects, the current distribution between parallel transistors is not equal, resulting in unequal power dissipation and potentially damaging the transistors. Therefore, power transistors are not easily paralleled compared to other types of transistors.

2. Susceptible to secondary breakdown:
Secondary breakdown is a phenomenon that occurs when a transistor is subjected to a high current and voltage simultaneously. It leads to localized heating and can cause the breakdown of the transistor. Power transistors, especially bipolar junction transistors (BJTs), are more susceptible to secondary breakdown compared to other types of transistors such as metal-oxide-semiconductor field-effect transistors (MOSFETs). This is due to the inherent structure and characteristics of BJTs.

3. Low transconductance than MOSFET:
Transconductance is a measure of the ability of a transistor to amplify a signal. It represents the change in output current with respect to the change in input voltage. MOSFETs generally have higher transconductance compared to power transistors, especially when operated in the saturation region. This is because MOSFETs have a higher input impedance and do not suffer from the current gain limitations of BJTs.

4. Negative temperature coefficient on collector current:
The collector current of a power transistor typically decreases with an increase in temperature. This negative temperature coefficient arises due to the temperature-dependent variations in the transistor's characteristics, such as the saturation current and the base-emitter voltage. This behavior is often desirable in power transistors as it helps to improve their stability and thermal management.

5. Low switching speed than MOSFET:
Switching speed refers to the ability of a transistor to transition between its on and off states quickly. MOSFETs generally have faster switching speeds compared to power transistors. This is because MOSFETs have a lower gate capacitance and do not suffer from the minority carrier storage time effects found in BJTs. Power transistors, on the other hand, have slower switching speeds, which can limit their use in high-frequency applications.

Conclusion:
In conclusion, a power transistor possesses the characteristics of not being easily paralleled, being susceptible to secondary breakdown, and having lower switching speed compared to MOSFETs. These characteristics make power transistors suitable for handling high-power applications where high current and voltage levels are involved.