The Gross Enrolment Ratio (GER) measures the number of students enrolled in higher education as a percentage of the eligible population aged between:

The correct option is 18 to 23 years.

The Gross Enrolment Ratio (GER) in higher education is a statistical measure used to determine the level of participation in higher education. It is calculated by dividing the total number of students enrolled in higher education (regardless of their age) by the total population of the age group that officially corresponds to this level of education, which is standardly defined as 18 to 23 years, and multiplying the result by 100.

Thus, the eligible population age group considered for calculating the GER in higher education is 18 to 23 years.