The activism of state citizens, who have demanded safer road condition

Split #1: activism, singular, needs the singular verb has led

Eliminate options A and B because both incorrectly use have led.

Split #2: amount versus number

amount is used for uncountable nouns, this way:
-- a significant amount of water evaporated
-- the amount of pollution decreased because of strict carpooling laws

number is used for countable nouns, this way
-- the number of traffic accidents decreased

Options B and E incorrectly use amount of traffic accidents.
Number of traffic accidents is correct.
Eliminate E (B already eliminated)

Split #3 : Meaning and causality

(C) "X [b]has led to a decrease in the number of Y" is clear.

X caused the decrease.

(D) "X has been significant in the decrease in the number of Y" is not as clear as (C).

-- What does "has been significant" mean?
-- Was activism literally was in the middle of [was significant in] the decrease

-- "Has been significant" does not make a straightforward causal connection between activism and the decrease.
-- The verb phrase "has been (significant) in" is not as strong or logically clear as "has led to."

-- Compare to the clear and direct logic in (C), which says X has led to Y.

To describe something as being significant in" does not establish direct causality, and certainly does not express causality as well as Option (C)

The correct answer is C