The correct answer is Nanoparticle.

• A particle having at least one dimension less than 10^-7  metre, is known as Nanoparticle (10^-9).
• At least one means 10^-8,10^-9,10^-10,10^-11, etc.
• Hence as per given options, 10^-9 is Nanoparticle.

• Milli particle-10^-3
  • ^​The millimetre or millimetre is a unit of length in the metric system, equal to one-thousandth of a metre, which is the SI base unit of length. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre. One millimetre is equal to 1000 micrometres or 1000000 nanometres​.
• Micro particle-10^-6
  • ^​The micrometre or micrometre, also commonly known as a micron, is an SI derived unit of length equalling 1×10⁻⁶ metre; that is, one-millionth of a metre. The next smallest common SI unit is the nanometre, equivalent to one one-thousandth of a micrometre, or one billionth of a metre.
• Nano particle-10^-9
  • ^​A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibres and tubes that are less than 100 nm in only two directions
• Pico particle-10^-12​
  • The picometre or picometer is a unit of length in the metric system, equal to 1×10⁻¹² m, or one trillionth of a metre, which is the SI base unit of length.
  • The picometre is one-thousandth of a nanometre, one-millionth of a micrometre, and one trillionth of a meter.
  • The symbol μμ was once used for it.