A transistor has collector current I_C = 10 mA and base current I_B = 40 mA. Compute the DC current gain β = I_C / I_B and identify the correct option.

Introduction / Context:
DC current gain for a BJT in common-emitter configuration is β = I_C / I_B. It is dimensionless, because both I_C and I_B are currents. This question checks unit awareness and the ability to compute β directly from given currents.

Given Data / Assumptions:

• I_C = 10 mA.
• I_B = 40 mA (notably large for a base current, but we use it as stated).
• β is defined as the ratio of these currents and therefore has no unit.

Concept / Approach:

• Compute β = I_C / I_B.
• Ensure consistent units (both in mA), and remember β is unitless.
• Compare the computed numeric value with the provided options and check units/formatting.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• 250 μA, 350 μA, 0.25 μA, 300 μA: These include current units and therefore do not represent a unitless gain. Numerically they also do not equal 0.25 as a pure ratio.
• None of the above: Correct, because the proper answer is the dimensionless value β = 0.25, not a current in microamperes.

Common Pitfalls:

• Treating β as if it had units (e.g., confusing it with a current magnitude).
• Mishandling unit conversions between mA and A.

Final Answer: