What is a niche?
a. the particular set of resources a species uses
b. a mating system in which one male mates with several females
c. an example of population cycling
d. an evolutionary arms race
e. a plant structure containing many flowers

a. The particular set of resources a species uses

Explanation:

A niche is described as the interaction of an individual with its environment. It is a functional position of an individual. The niche includes how an organism lives, its behavior and abiotic factors such as light, temperature, climate etc.The ecological niche is a multidimensional thought which includes the role of an individual like predation, mutualism, parasitism, competition etc. An example of niche; A bird lives on a tree and eat the fruits.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.