A constant value of $\phi$ results in a linear Mohr envelope.

For actual soils under a wide range of stresses, the Mohr envelope is curved rather than straight.

Assuming $\phi$ is constant is a common simplification used in conventional soil mechanics.

Simplifies analytical calculations for slope stability and bearing capacity.

Provides a close approximation for standard structural load stress ranges.

Overestimates shear strength at very high normal stress levels.

Ignores stress-dependency of the internal friction angle.

Design of shallow and deep foundations.

Stability analysis of earth retaining structures and embankments.

None of the numerical values or ranges given in Options A, B, or C ($90°$, $>90°$, or $<90°$) specifically define the condition for a straight line; the requirement is that $\phi$ is assumed to be constant.

Angle of internal friction $\phi$ typically ranges between $0°$ and $45°$ for real soils.