Which of the following remains unchanged on descending a group in the periodic table?  

Modern periodic table states that the physical and chemical properties of an element of periodic function of its atomic number. There are 18 groups and 7 periods which are distributed in four blocks namely s, p, d and f. In the modern periodic table group number 1 to 2 in s block, group number 13 to 18 are in p block, group 3 to 12 are in the d block and 14 elements are in f block. In general, the electrical arrangement of an element's outer shell is comparable within a group. For elements that are members of the same group, the valence shell's overall electrical arrangement is the same. Valence electrons are the electrons that are found in valence shells. The electrical arrangement of the valence shell often stays the same as you move down the group. The size, density, and metallic nature of the atoms typically change as we progress through a period or a group. The outermost or final shell of any element in which electrons are contained is called the valence shell. At each succeeding step as we descend the group, one shell is added, but the final shell's electron count stays constant. This is due to the fact that the atomic number follows a certain Trend that occurs again when elements belong to the same group. Hence, the correct option is option A.