C

Step 1: The unit of magnetic flux is defined as the product of the magnetic field (in Teslas) and the area (in square meters) through which the magnetic field lines pass.
Step 2: In the SI (International System of Units), the unit of magnetic flux is the Weber (Wb), which is equivalent to one Tesla meter squared.
Step 3: In the CGS (Centimeter-Gram-Second) system, the unit of magnetic flux is the Maxwell, which is less commonly used in modern applications.
Step 4: The MKS (Meter-Kilogram-Second) system is essentially the same as the SI system, as SI is based on the MKS system but incorporates additional standardized units.
Conclusion: Since the primary and widely accepted unit for magnetic flux in modern usage is the Weber (found in the SI system), the correct answer is C (SI).