‘SONAR’ emits which of the following waves

Which of the following emits by tritium-

Which of the following will emit positron?

Work function of a metal is 2.1 eV . Which of the waves of the following wavelengths will be able to emit photoelectrons from its surface ?

Which of the following principles is being used in Sonar Technology?

Which of the following principles is being used in sonar technology ?