A metric sequence of length units is given as Millimetre, Micrometre, Nanometre, ?. Choose the correct term from the options that logically completes this sequence of metric prefixes from larger to smaller units.

Introduction / Context:
This question tests your knowledge of metric prefixes used in science and mathematics. The terms millimetre, micrometre, nanometre, and picometre correspond to specific powers of ten, and they appear frequently in physics, chemistry, and engineering. Recognising the correct order of these prefixes helps in unit conversion and interpretation of extremely small measurements.

Given Data / Assumptions:

• The given sequence is Millimetre, Micrometre, Nanometre, ?
• All terms refer to metric length units.
• We assume the standard scientific meaning of prefixes milli, micro, nano, and pico.
• We must select the next smaller unit in the progression of metric prefixes.

Concept / Approach:
Metric prefixes represent powers of ten. For length:

• Milli means 10^-3 of a metre.
• Micro means 10^-6 of a metre.
• Nano means 10^-9 of a metre.
• Pico means 10^-12 of a metre.

Each step in the given series moves to a unit that is 10^-3 times the previous one, that is one thousand times smaller in linear size.

Step-by-Step Solution:
Step 1: Interpret Millimetre as 10^-3 m. Step 2: Interpret Micrometre as 10^-6 m, which is 10^-3 times a millimetre. So the sequence is getting smaller. Step 3: Interpret Nanometre as 10^-9 m, which again is 10^-3 times a micrometre. The pattern is consistent. Step 4: The next logical step should then be another factor of 10^-3 smaller than a nanometre, that is 10^-12 m. Step 5: The metric prefix for 10^-12 is pico. Therefore, the next term in the sequence must be Picometre.

Verification / Alternative check:
An alternative way is to recall the common order of decreasing prefixes used in nanotechnology and physics: millimetre, micrometre, nanometre, picometre, femtometre, and so on. From this ordered list, it is clear that picometre follows nanometre.

Why Other Options Are Wrong:
Kilometre corresponds to 10^3 m, which is much larger than a metre and thus breaks the decreasing size pattern. Metre is the base unit (10^0 m) and is larger than all the preceding units in the sequence. Centimetre is 10^-2 m, which is larger than millimetre and does not align with the progression toward smaller dimensions. Decimetre is 10^-1 m, also larger than millimetre, and again does not fit the logical order of smaller and smaller units.

Common Pitfalls:
Candidates sometimes confuse the order of very small prefixes such as nano and pico, or mix them up with prefixes used for large values. Memorising these as an ordered chain from larger to smaller and from smaller to larger helps reduce such errors. Also, thinking in terms of powers of ten supports quick reasoning in exam conditions.

Final Answer:
The correct term that continues the decreasing metric sequence Millimetre, Micrometre, Nanometre is Picometre.