Explanation

Newton metre (Nm) is a unit of torque, but it is fundamentally derived from units of force times distance, making it dimensionally related to work and energy. However, the base unit for length is the metre (m), while Newton-metre is a composite unit. Option B represents the fundamental dimension that forms part of this unit. Option A is incorrect because substance has units like moles or mass per volume. Option C is incorrect because torque *is* measured in Newton-metre, but the question asks what 'Newton metre' *is a unit of*, implying its base physical quantity. Option D is incorrect because work is also measured in Joules (J), which is equivalent to N*m.