Answer:
X - rays
Explanation:
One astronomer is studying the nature of an electromagnetic wave emitted by hot gases in the universe.
We know that frequency is defined as the number of oscillations or vibrations per unit time.
We have to write name of the electromagnetic wave whose frequency is higher than visible light but lower than gamma rays.
Frequency of visible light is 
Frequency of gamma rays is more than 
The frequency of X-rays lies between visible light and gamma rays i.e. the frequency of X - rays is in between 
Hence, the correct option is (A) " X - rays ".