Maxwell argued that a magnetic field is generated not only by a current but also by a:

1. A steady current makes a magnetic field, which was already known. 2. Maxwell claimed a second source of magnetic field exists. 3. Not only an electric current but also a time-varying electric field generates a magnetic field. 4. So the answer is a time-varying electric field. _Source: NCERT Class XII Physics Part I, Ch 8 'Electromagnetic Waves'_