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(B) When current $I$ flows through a conductor of varying cross-section,the current remains constant through every cross-section due to the continuity

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the magnitude of the electric field increases

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(B) When current $I$ flows through a conductor of varying cross-section,the current remains constant through every cross-section due to the continuity of charge.Since $I = jA$,where $j$ is the current density and $A$ is the cross-sectional area,we have $j_1 A_1 = j_2 A_2$.As the area decreases $(A_2  j_1)$.Also,current density is related to drift velocity $v_d$ by $j = n e v_d$. Since $n$ and $e$ are constant,the drift velocity $v_d$ increases as the area decreases.From Ohm's law in microscopic form,$j = \frac{E}{\rho}$,where $E$ is the electric field and $\rho$ is the resistivity of the material.Since $j$ increases and $\rho$ is constant for a given material,the magnitude of the electric field $E$ must increase in the narrowing region.Therefore,the correct option is $(b)$.

$A$ steady current $I$ is set up in a wire whose cross-sectional area decreases in the direction of the flow of the current. Then,as we examine the narrowing region,

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