Answer:
each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.
"some elements have only one stable isotope"
Explanation:
The correct answer is E. C6H6 is not soluble in water. This is determined by the polarities of these substances. Benzene is a non-polar substance because it has equal number of electrons sharing a bond thus it does not dissolve in water which is a polar substance.
Answer: Option C is correct.
Explanation: Average kinetic energy is directly proportional to the absolute temperature. Higher the temperature means higher the kinetic energy.
Average kinetic energy is given by:

Where, k = Boltzman constant
T = Temperature
We are given different temperatures, so to compare they all should have the same units.
a) 298K
b) 267K
c) 27°C = 273+27 = 300K
d) 12°C = 273+12 = 285K
Looking at the temperature values, C part will have the highest average kinetic energy.
The most common method to get an idea about the pH of solution is to use an acid base indicator. An indicator is a large organic molecule that works somewhat like a " color dye". Whereas most dyes do not change color with the amount of acid or base present, there are many molecules, known as acid - base indicators<span> , which do respond to a change in the hydrogen ion concentration. Most of the indicators are themselves weak acids.</span>