Which of the following halides is the most stable?

In an emission spectrum,the transitions $n_2 \to n_1, n_3 \to n_1, n_4 \to n_1, n_5 \to n_1$ result in the emission of energies $E_1, E_2, E_3, E_4$ respectively. What is the increasing order of these energies?

Two resistances of $400 \ \Omega$ and $800 \ \Omega$ are connected in series with a $6 \ V$ battery of negligible internal resistance. $A$ voltmeter of resistance $10,000 \ \Omega$ is used to measure the potential difference across the $400 \ \Omega$ resistor. The error in the measurement of the potential difference in volts is approximately:

What is the $IUPAC$ name of the following compound?

$A$ car is moving with a speed of $72 ~km/h$ towards a hill. The car blows its horn at a distance of $1800 ~m$ from the hill. If the echo is heard after $10 ~s$,what is the speed of sound in $m/s$?

Ethanol,when reacted with $PCl_5$,gives $A$,$POCl_3$,and $HCl$. $A$ reacts with silver nitrite to form $B$ (major product) and $AgCl$. $A$ and $B$ are respectively: