The probability would be 1/4 or 25%
Assuming that curly hair is represented by A and straight hair by a.
Crossing two heterozygous parents:
Aa x Aa
AA Aa Aa aa
Offspring:
AA - curly hair
Aa - curly hair
aa - straight hair
Probability of curly hair = 3/4
Probability of straight hair = 1/4
More on monohybrid crossing can be found here: https://brainly.com/question/1185199