Which of the following pair of compound – melting point is correct?
I. Acetic acid – 290K
II. Ethanol – 156K

The correct option is Both I and II.

Let us analyze the given compounds and their respective melting points step-by-step:

I. Acetic acid (Ethanoic acid):
Acetic acid has the chemical formula:
${\mathrm{CH}}_3\mathrm{COOH}$
The melting point of pure (glacial) acetic acid is approximately 16.6 °C. To convert this temperature to Kelvin (K), we use the relation:
$\text{Temperature in Kelvin (K)}=\text{Temperature in Celsius (°C)}+273.15$
Substituting the value:
$16.6+273.15=289.75\text{K}$
This is rounded to 290 K in standard textbooks. Thus, the first statement (Acetic acid – 290K) is correct.

II. Ethanol:
Ethanol has the chemical formula:
$C_2{H}_5\mathrm{OH}$
The melting point of ethanol is extremely low, at approximately -114 °C (or 159 K). In school textbooks (such as NCERT Science), the melting point of ethanol is officially documented as 156 K (approximately -117 °C). Thus, the second statement (Ethanol – 156K) is also correct.

Since both pairs of compounds and their melting points are correct, the correct answer is Both I and II.