1. Capillary rise formula

For a liquid rising in a capillary tube,

$h = \frac{2T\cos\theta}{\rho r g}$

where:

• $h$ = height of capillary rise
• $T$ = surface tension
• $\theta$ = angle of contact
• $\rho$ = density of liquid
• $r$ = radius of capillary
• $g$ = acceleration due to gravity

So, capillary rise is inversely proportional to gravity:

$h \propto \frac{1}{g}$

2. In a freely falling elevator

When the whole arrangement is in a freely falling elevator, the effective gravity becomes

$g_{\text{eff}} = 0$

Therefore, from the formula,

$h = \frac{2T\cos\theta}{\rho r g_{\text{eff}}} \to \infty$

This means the liquid is no longer limited by gravity and will keep rising until constrained by the tube length.

3. Given tube length

The capillary tube is only $20\,\text{cm}$ long. So the maximum possible length of water column inside it is the full tube length:

$20\,\text{cm}$

4. Option check

• A: $10\,\text{cm}$ — incorrect
• B: $8\,\text{cm}$ — incorrect
• C: $20\,\text{cm}$ — correct
• D: $4\,\text{cm}$ — incorrect

Hence, the correct answer is C.