Answer: Option A. -> eka-aluminium ($E_a$ ) 
:
A, B, and D
When Mendeleev proposed his periodic table, he noted gaps in the table and predicted that unknown elements exist with properties appropriate to fill those gaps. The four predicted elements- eka-boron ($E_b$ ), eka-aluminium ($E_a$ ), eka-manganese ( $E_m$) and eka-silicon ($E_s$ ) proved to be good predictors of the properties of scandium, gallium, technetium and germanium respectively, with each filling the spot in the periodic table assigned by Mendeleev.

Answer: Option B. -> ${}_{12}$Mg
:
B

Alkaline earth elements are elements found in the second group of the periodic table. These elements have two valence electrons.
In the given options magnesium $\text{(Mg)}$ is the only element having two valence electrons. So it belongs to the second group and is an alkaline earth metal.

Answer: Option C. -> chalcogens
:
C
Elements in the ${16}^{th}$ group of the modern periodic table are also known as chalcogens. The ${16}^{th}$ group consists of the elements, oxygen (O), sulphur (S), selenium (Se), tellurium (Te), and the radioactive element polonium (Po). They are the family of oxygen and can be found in copper ores.

Answer: Option C. -> The mass of middle element was roughly the average of atomic masses of other two elements.
:
A, C, and D
Dobereiner stated that when elements with similar properties are arranged in a group of three (triad) in the increasing order of their atomic mass, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements. 
However, similar elements occur at an interval of seven elements is one of the feature of Newland's octave.