Answer:- Frequency is
.
Solution:- The wavelength of electromagnetic radiation is given as 625 nm and we are asked to calculate the frequency. Frequency is inversely proportional to the wavelength and the formula used to calculate frequency when wavelength is given is:

where,
is frequency, c is speed of light and
is the wavelength.
Value of speed of light is
. We need to convert the given wavelength from nm to m.

So, 
= 
Let's plug in the values in the formula and calculate the frequency:

=
or Hz.
So, the frequency of the electromagnetic wave is
.
Answer:
D. Acidic salt
Explanation:
Acidic salts:
Salts formed by incomplete neutralisation of poly-basic acids are called acidic salts. Such salts still contain one or more replaceable hydrogen atoms. These salts when neutralised by bases form normal salts.
Answer:
There are often not more than one or two independent variables tested in an experiment.
An ion has a positive or negative charge. While a covalent bond always has no charge, meaning it is neutral.