The negative feedback in an amplifier

Identify the semiconductor devices whose characteristics are given below,in the order $(a), (b), (c), (d)$.

The box in the circuit below has two inputs marked $V_{+}$ and $V_{-}$ and a single output marked $V_{o}$. The output obeys the following relation:
$V_{o} = \begin{cases} +10 \, V & \text{if } V_{+} > V_{-} \\ -10 \, V & \text{if } V_{+} < V_{-} \end{cases}$
Which of the following graphs represents the output $V_{o}$ as a function of time $t$?

In an oscillator,if $\beta$ is the feedback factor and $A$ is the gain of the amplifier,then sustained oscillations are obtained when:

The correct curve between voltage gain $({A_v})$ and load resistance $({R_L})$ is

Select the correct statements from the following: