In the International System of Units (SI), how many base or fundamental units are defined in total?

Introduction / Context:
The International System of Units, commonly called the SI system, is the globally accepted standard for scientific and engineering measurements. It is built on a small set of fundamental or base units from which all other derived units are constructed. This question checks your memory of how many such base units exist in the modern SI system, which is important whenever you study dimensional analysis, unit conversion, or physical formulas.

Given Data / Assumptions:

• We are talking about the official SI system, not older metric systems.
• We need the count of base or fundamental units, not derived units.
• The latest definition of the SI base units is assumed.

Concept / Approach:
The SI system defines a fixed set of base quantities, each with a corresponding base unit. These include length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Their units are metre, kilogram, second, ampere, kelvin, mole, and candela. All other physical quantities such as force, energy, or pressure are expressed in terms of these base units through derived units like newton, joule, or pascal.

Step-by-Step Solution:
Step 1: List the seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Step 2: Match each quantity with its SI base unit: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd). Step 3: Count the number of base units in this list. There are exactly seven distinct base units. Step 4: Conclude that the number of base or fundamental SI units is 7.

Verification / Alternative check:
You can verify this information from any standard physics or engineering reference, where the SI units table is printed in the introductory chapters. It always lists the same seven base units. Although their definitions have been refined using physical constants like the speed of light or Planck constant, the number of base units itself has remained seven in the modern SI system.

Why Other Options Are Wrong:
4: This is too small and would not be enough to describe all fundamental physical quantities. 6: This misses one of the standard base quantities and therefore is incorrect. 5: This also underestimates the total number of SI base units.

Common Pitfalls:
Learners sometimes confuse base units with derived units and may think quantities like force or energy are fundamental. Another common mistake is to forget either mole or candela when mentally counting the list, leading to an incorrect number such as 5 or 6. To avoid this, it is helpful to remember a simple phrase or mnemonic for all seven basic quantities and their symbols.

Final Answer:
The International System of Units defines seven base or fundamental units. Therefore, the correct answer is 7.