The prefix zepto stands for:
A.\[{10^9}\]
B.\[{10^{ - 12}}\]
C.\[{10^{ - 15}}\]
D.\[{10^{ - 21}}\]

Hint: Zepto is a prefix used for the measurement of length. It is a SI unit of measurement indicated by the symbol z.
\[{\rm{1 \ zeptometre = 1}}{{\rm{0}}^{{\rm{ - 21}}}}{\rm{m}}\]

Complete Step by Step Solution:
Every material has distinctive characteristic properties. These properties can be divided into two types.
1. Physical properties like colour, odour, melting point, boiling point, density, etc.,
2. Chemical properties such as composition, combustibility, activity with acids and bases, etc.

Physical properties can be calculated or observed without altering the individuality or the composition of the substance. The measurement or observation of chemical properties needs a chemical change to happen. The measure of physical properties doesn't need the happening of a chemical change.

Chemists describe, analyse and foresee the behaviour of substances established on an understanding of their physical and chemical properties which are specified by careful measurement and experiments.

Measurement of physical properties like length, area, volume, etc., are quantitative.
Any quantitative observation or measurement is depicted by a number and then the units
in which it is calculated. For example, 10 m where 10 is the number and m indicates the metre, the unit in which the length is calculated. The metre is the SI unit for length measure. The International System of Units abbreviated as SI has prefixes for units of measurement. Zepto is one of them.
\[{\rm{1zeptometre = 1}}{{\rm{0}}^{{\rm{ - 21}}}}{\rm{m}}\]
So, option D is correct.

Note: The International System of Units (in French Le Système International d’Unités was founded by the 11^th General Conference on Weights and Measures (CGPM from Conférence Générale des Poids et Mesures). The SI system has seven base units. These units pertain to the seven basic scientific quantities which include the units of length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous Intensity.