There are about 1.6 million known species, most of which are
a. animals
b. plants
c. insects
d. humans

The answer is A. Animals. 

First of all humans and insects belong to the group of animals. There are about 1.6 million known species of animals which include many more groups. At least 1 million of animal species are insects. Humans belong to the one only species Homo sapiens of Hominina clade. Talking about plants, there are about 300.000 species, leaving animals as the right answer.

the glands which are associated with each description is given below:

pancreas - is an endocrine and exocrine gland that secretes juices with enzymes for digestion. salivary glands- with protection and digestion in the mouth. liver- secretes bile for digestion.   lachrymal glands - secrete tears to moisten eyes and protect them from particles and infection.   sweat gland -regulates water, salt, and temperature. oil glands - to form a protective barrier on the skin.   goblet cells - secrete mucus for protection.