In this question, we are given that chlorine occurs in nature in two isotopic forms of masses 35 u and 37 u in the ratio of 3:1 respectively. Isotopes are atoms of the same element with different mass numbers. The average atomic mass of an element takes into account the isotopic masses and their respective abundances.

In this case, we have two isotopes of chlorine, with masses of 35 u and 37 u. The ratio of their abundances is given as 3:1. To find the average atomic mass, we need to calculate the weighted average of these isotopes using their masses and abundances.

To do this, we multiply the mass of each isotope by its abundance and then sum up these values. In this case, it would be (35 u * 3) + (37 u * 1), which equals 105 u + 37 u = 142 u.

Finally, we divide the sum by the total number of isotopes, which is 4 in this case (3+1), to get the average atomic mass. Therefore, the average atomic mass of chlorine is 142 u / 4 = 35.5 u.

Now, looking at the options provided, we can see