What was discovered in 1965 and is believed to be leftover thermal energy from the big bang?

Cosmic Microwave Background (CMB) radiation was discovered in the year 1965

Explanation:

The Cosmic Microwave Background (CMB) are a type of electromagnetic radiation which were believed to be the remnant of thermal energy from the big bang. Soon after the big bang explosion, the universe started to form and the CMB radiations were extensively radiated in all the direction. The universe then started to inflate rapidly and expanded. This expansion of the universe is still continuing. The CMB is very important because it helps in determining how the universe started to form in its initial time. These CMB radiations describes about the composition as well as the age of the universe.

gametes or germ cells are haploid cells (example: sperm and ova) containing only one set (or n) number of chromosomes and autosomal or somatic cells are diploid cells containing 2n number of chromosomes.

the answer is d. chordata