'Stratum' is a layer of rock in the ground.
Option A: The plural form of 'stratum' is 'strata'.'Strata' are 'series of layers of rock in the ground'.
Hence option A is correct.
Option B: 'Strati' is the plural form of 'stratus' and not 'stratum'.
'Stratum' refers to a layer of ground whereas 'stratus' refers to a form of cloud.
Hence option B is wrong.
Option C: 'Stratums' is not the plural form of 'stratum'. Also, the word 'stratums' does not exist in standard English language.
Hence option C is wrong.
Option D is wrong because the answer lies in A.