c and d
In c, there are 3 tulips and 4 daffodils. In d, make groups of 2 flowers, and there are 3 groups of tulips and 4 groups of daffodils.

a and b
The phrase "of the flowers" tells you that the whole is the total number of flowers. In a, there are 6 tulips and 8 total flowers: 6/8=3/4. In b, there are 9 tulips and 12 flowers: 9/12=3/4

c and d
The phrase "as many (tulips) as (daffodils)" tells you that the denominator corresponds to the number of daffodils and the numerator is the number of tulips.

a and b
the phrase "of the flowers" means you are looking at a part-whole relationship where the whole is the total number of flowers.

The ratio of turtles to frogs is 2:3
or
The ratio of frogs to turtles is 3:2.

There are 2/3 as many turtles as frogs.
or
There are 1 1/2 times as many frogs as turtles.
Notice that while it would be mathematically correct to say 3/4 times as many frogs as turtles, that the standard English phrase uses a mixed number.

2/5 of the animals are turtles.

40% of the animals are turtles.