Use the periodic table of elements to determine which of the options below show the two MOST similar elements according to their placement in the table.

To determine which pair of elements is the most similar according to their placement in the periodic table, we should look for elements that are in the same group (column) or period (row).

Here's a brief overview of the options:

1. **Li (Lithium) & Mg (Manganese)**: These elements are in different groups and periods, so they are not very similar.

2. **Li (Lithium) & Be (Beryllium)**: These two elements are both in group 1 and group 2 respectively, in the same period as **Li** is in Period 2 and **Be** is also in Period 2. They are both metals and have similar properties, but they are not in the same group.

3. **C (carbon) & O (oxygen)**: These elements are in the same period (Period 2) but are in different groups (Carbon is in Group 14, and Oxygen is in Group 16). They are not very similar chemically.

4. **Mg (Manganese) & Ca (Calcium)**: These two elements are in different periods and groups, so they aren't very similar either. - Mg is in Group 2 (alkaline earth metal) in Period 3, and Ca is in Group 2 in Period 4.

Based on this analysis, the pair of elements that are closest in similarity is **Li (Lithium) & Be (Beryllium)**. Thus, the correct answer would be:

**Li (Lithium) & Be (Beryllium)**