Question

The fusion of two deuterium nuclei requires extreme temperature and _____.

Answer 1

The answer is A. high pressure. The fusion of two deuterium nuclei can only be done with high temperature and also highpressure to prevent electrostatic repulsion of the proton. The successful combination of the deuterium then creates helium as it all combines.
    Source: Objective Pre-Engineering Physics, Objective Questions with a Glossary of Knowledge wrote by Shobhana Sharma/ Google books

Answer 2

One of the stable isotopes of hydrogen is deuterium. The nucleus of the deuterium has one proton and one neutron. The fusion of the two nuclei of the deuterium requires certain conditions, which includes high temperature and pressure. Such extreme conditions are required to overcome the repulsion of the protons in the each nucleus.

Hence, the correct answer is option (a) - high pressure.