An ideal liquid flows through a horizontal tube of variable diameter. The pressure is lowest where the _____.

Using Bernoulli's theorem, which states the total mechanical energy, potential energy and kinetic energy of an ideal and steady fluid flow remain conserved at a section during flow process.

$\Rightarrow \frac{P}{\rho }+\frac{v^2}{2}+gh=\text{constant}$

$P$is the pressure of fluid at the section, $v$is the velocity of fluid at that section and $h$is the height of fluid flow.

$\frac{P}{\rho }\to$pressure head

$\frac{v^2}{2}\to$velocity head

$gh\to$elevation head

According to Bernoulli's theorem, total head remain conserved between the two section of continuous fluid flow in the pipe.

Therefore, if the pressure is the lowest then the velocity of the fluid will be maximum there considering the fact that height of pipe is same.