Correct Answer: A plant's root grows downward

Solution : According to the above-given statement –
First option: I am a botanist.
A botanist refers to a person who studies plants. From the above statement, it can be said that anybody can observe the plants. It's not necessary to become a botanist to observe the plant.

Second option: Some plants' roots grow downward.
It's clear from the above statement that all plants' roots grow downwards not some. Hence, it's incorrect.
Third option: A plant's root grows downward.
This option proves the above statement correct as it states that a plant's root grows downward.
Fourth option: Most of the plants' roots grow downward.
This option states about most of the plants' roots but, according to the statement, it's clear that all plants' roots grow downward.

Therefore, from the above, we can say that the third option satisfies the statement. Hence, the third option is correct.