According to the question the alkene ‘A’ on ozonolysis gives a mixture of ethanal and pentan-3- one. This could be represented as follows: -

Now, we know that during the process of ozonolysis, an ozonide having a cyclic structure is formed as an intermediate which undergoes cleavage to give the final products. Ethanal and pentan-3-one are obtained from the intermediate ozonide. Hence, the expected structure of the ozonide could be:-

This ozonide is being formed as an addition of ozone to 'A'. The desired structure of 'A' can be obtained by the removal of ozone from the ozonide. Hence, the structural formula of 'A' is:-

3-Ethylpent-2-ene