Because there are 64 possible codons and only 20 amino acids:
a. many mutations have no effect on phenotype.
b. every mutation has some effect on phenotype.
c. each trna molecule contains two extra codons.
d. each trna molecule contains two extra amino acids.

Option A

Explanation:

There are 64 possible codons, out of which 61 codons code for particular amino acid and three codons are stop codons. As there are 61 codons for 20 amino acids, some of the amino acids are coded by more than one codons.

Due to this many mutations cannot show their phenotypic effect. For example, if an original codon UUA  gets converted into UUG due to a point mutation, there would be no noticeable change seen in the phenotype of the organism because both of the codons code for the same amino acid, leucine.

a fossil fuel is a fuel formed by natural processes, such as anaerobic decomposition of buried dead organisms, containing energy originating in ancient photosynthesis.[1] the age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years.[2] fossil fuels contain high percentages of carbon and include petroleum, coal, and natural gas.[3] other commonly used derivatives include kerosene and propane. fossil fuels range from volatile materials with low carbon to hydrogen ratios like methane, to liquids like petroleum, to nonvolatile materials composed of almost pure carbon, like anthracite coal. methane can be found in hydrocarbon fields either alone, associated with oil, or in the form of methane clathrates.

the theory that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the earth's crust over millions of years was first introduced by andreas libavius "in his 1597 alchemia [alchymia]" and later by mikhail lomonosov "as early as 1757 and certainly by 1763".[4] the first use of the term "fossil fuel" was by the german chemist caspar neumann, in english translation in 1759.[5]

in 2017 the world's primary energy sources consisted of petroleum (34%), coal (28%), natural gas (23%), amounting to an 85% share for fossil fuels in primary energy consumption in the world.[6] non-fossil sources in 2006 included nuclear (8.5%), hydroelectric (6.3%), and others (geothermal, solar, tidal, wind, wood, waste) amounting to 0.9%.[7] world energy consumption was growing at about 2.3% per year. in 2015 about 18% of worldwide consumption was from renewable sources.[8]