Adihybrid cross is created from plants that are heterozygous for both round seeds and yellow seed color. what is the ratio of offspring that have round seeds and yellow seed color?
1: 16
3: 16
9: 16
16: 16

The answer is 9:16

Imagine that we have two genes - one for the shape and one for the color of the seed. Each gene has two different alleles (forms) - one recessive and one dominant that can mask the recessive one if found together:
A - a dominant allele for round seed
a - a recessive allele for wrinkled seed
B - a dominant allele for yellow seed
b - a recessive allele for green seed

Let's take a look at the possible genotypes for each gene:
AA - dominant homozygote with round seeds
Aa - heterozygote with round seeds
aa - recessive homozygote with wrinkled seeds

BB - dominant homozygote with yellow seeds
Bb - heterozygote with yellow seeds
bb - recessive homozygote with green seeds

Since the plant is heterozygous for both round seeds and yellow seed color, its genotype is AaBb (heterozygote for round seed is Aa, and heterozygote for yellow seed is Bb). 
After crossing two plants heterozygous for both round seeds and yellow seed color, you will get 9 plants with round seeds and yellow seed color of total 16 plants (take a look at the uploaded image of Punnett square (with yellow color are marked plants with round and yellow seeds) and possible genotypes above).