If the percentage of thymine is 30% what would the percentage of cytosine be

We use Chargaff's rule.

[A] + [G] = [C] + [T]

[A] + [G] + [C] + [T] = 100%

A is Adenine, G is Guanine, C is cytosine, T is Thymine.

Adenine always pairs with Thymine.

Guanine always  pairs with Cytosine

So if Thymine is 30% then  Adenine is 30%  → 60% total

Therefore 100% - 60% = 40%

40%  is shared equally among Cytosine and Guanine

Therefore Cytosine will be 20% and Guanine 20%

Cytosine is 20%

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.