Answer: Option A. -> Sonar
Sonar (originally an acronym for Sound Navigation And Ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, communicate with or detect objects on or under the surface of the water, such as other vessels.

Answer: Option D. -> They generate ultrasonic sound waves
Bats can fly in dark because they produce ultrasonic waves. ... It produces ultrasonic waves which strike with the obstacles present on its way.

Answer: Option B. -> Silver
In a spherical mirror, one of the two curved surfaces is coated with a thin layer of silver followed by a coating of red lead oxide paint. Thus, one side of the spherical mirror is opaque and the other side is a highly polished reflecting surface.

Answer: Option D. -> Total internal reflection
If the angle of incidence of light ray in the denser medium is greater than the critical angle of the concerned media pair, the light is not at all reflected in to the rarer medium but is totally reflected. This is known as 'total internal reflection'.

Answer: Option A. -> Audible sounds
The sounds having a frequency of 20 Hertz to 20,000 Hertz are known as audible sounds.

Answer: Option C. -> Steam
Steam produces more severe burns than boiling water even though both are at 100oC because steam contains more heat, in the form of latent heat, than boiling water.

Answer: Option B. -> Latent heat
When air is cooled and water vapour condenses to form water liquid, energy is released as water particles 'give up' some of their energy in order to slow down and move closer together again. The same is true when liquid water freezes to become ice. This energy is also called latent heat.

Answer: Option C. -> Float
Adding salt to the water increases the density of the solution because the salt increases the mass without changing the volume very much. When enough salt is added to the water, the saltwater solution's density becomes higher than the egg's, so the egg will then float.

Answer: Option B. -> Water
Heat capacity (usually denoted by a capital C, often with subscripts), or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance’s temperature by a given amount. In the International System of Units (SI), heat capacity is expressed in units of joule(s) (J) per Kelvin (K). Among iron piece, water, gold piece and benzene; water has the maximum heat capacity i.e. 4.1813 j/k.

Answer: Option B. -> Steam has latent heat
Steam produces more severe burns than boiling water even though both are at 100oC because steam contains more heat, in the form of latent heat, than boiling water.