Step 1: Understanding the Question:
The question asks which of the classical laws of electromagnetism was modified by James Clerk Maxwell to formulate his set of four fundamental equations.
Step 2: Key Concept and Approach:
Maxwell identified an inconsistency in Ampere's Circuital Law when applied to circuits with time-varying electric fields, such as a charging capacitor.
To resolve this, he introduced the concept of displacement current, modifying Ampere's law into the Ampere-Maxwell Law.
Step 3: Detailed Explanation:
• Inconsistency in Ampere's Law:
Ampere's Circuital Law is written as:
\[ \oint \vec{B} \cdot d\vec{l} = \mu_0 I_c \]
During the charging of a capacitor, a magnetic field exists between the plates, but no conduction current ($I_c$) flows there, causing a mathematical contradiction.
• Maxwell's Correction:
Maxwell proposed that a changing electric field between the plates induces a magnetic field, acting as a virtual current called the displacement current ($I_d$):
\[ I_d = \varepsilon_0 \frac{d\Phi_E}{dt} \]
He modified Ampere's law to include this term:
\[ \oint \vec{B} \cdot d\vec{l} = \mu_0 \left( I_c + \varepsilon_0 \frac{d\Phi_E}{dt} \right) \]
This modified equation is called the Ampere-Maxwell Law.
Step 4: Final Answer:
The law modified by Maxwell is Ampere's circuit law, which corresponds to Option (D).