an open vessel containing water is given a constant acceleration in horizontal directionthen the free surface of water gets sloped with horizontal at an angle X given by
cosX= g/a
tanX=a/g
sinX=a/g
tanX=g/a

Dear Student,

There are two forces are acted on the free surface of water.

1) Downward gravitational force = mg
2) F = ma


The resultant force R,  makes a right angle with the surface of water.

If we consider the angle theta which the resultant force makes with the gravitational force, we can write

$$\begin{gathered} \tan \theta =\frac{ma}{mg} \\ tan\theta =\frac{a}{g} \end{gathered}$$
Hence, the correct option is (B).

Please do post your queries in case you need any clarifications.

Regards