Step 1: Understand uniform acceleration.
Uniform acceleration means acceleration is constant with time.
Step 2: Relation between velocity and time.
From kinematics:
\[
v = u + at
\]
This shows velocity varies linearly with time.
Step 3: Analyze option (A).
A horizontal velocity-time graph indicates constant velocity, which means zero acceleration. Hence incorrect.
Step 4: Analyze option (B).
A horizontal displacement-time graph means position is constant (no motion), so acceleration is zero. Hence incorrect.
Step 5: Analyze option (C).
A straight line displacement-time graph represents constant velocity, not acceleration. Hence incorrect.
Step 6: Analyze option (D).
A straight line velocity-time graph shows velocity increasing linearly with time, indicating constant acceleration.
Step 7: Final conclusion.
\[
\boxed{\text{Velocity-time graph is a straight line}}
\]