U-235, the isotope of uranium commonly utilized in nuclear power plants, is

a. the most common of the naturally occurring isotopes of uranium
b. heavier than the other well-known isotope of uranium
c. rare even in uranium oxide deposits, and must be enriched before use as fuel
d. only found in granite plutons

Option C.

Explanation:

Natural uranium is composed by 99.27% of U-238, 0.72% of U-235 and 0.005% of U-234.

Of the isotopes of natural uranium, the U-235 is the most commonly utilized in nuclear power plants, but since it is only 0.72% of natural uranium it needs to be enriched before being used as fuel to sustain a nuclear fission reaction in nuclear power plants.    

The U-238 is the most common and the heaviest of the three naturally occurring isotopes of uranium.

So, from the above the correct option is the C.    

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C

Explanation:

Uranium is made up three isotopes which are U-234, U-235 and U-238. Among the three isotopes U-238 is the naturally occurring and U-235 is the least occurring.

U-235 is used as a fuel in nuclear reactors and also used in nuclear weapons as explosives. It is a very rare isotope of uranium and present in amounts of only 0.7% of naturally occurring uranium, hence its most treasured among the isotopes.

found in the nucleus

has mass of one amu