ANS:D - All the above.

The main assumption of Bernoulli's equation is: All the above. Each of the statements you provided represents a fundamental assumption that is inherent in Bernoulli's equation:

1. The velocity and energy of liquid particles across any cross-section of a pipe are uniform: This assumption implies that there are no significant changes in velocity or energy within the flow, and thus, the equation applies along a streamline where the flow is steady.
2. No external force except gravity acts on the liquid: This assumption means that the flow is not influenced by external forces such as friction, viscosity, or other non-conservative forces.
3. There is no loss of energy of the liquid while flowing: Bernoulli's equation assumes that the energy of the fluid remains constant along a streamline, implying that there are no energy losses due to friction, turbulence, or other dissipative processes.