Identify the alkane having two secondary carbon atoms.:

A secondary carbon atom is one that is directly bonded to exactly two other carbon atoms.

Let us analyze the given options:

1. Cyclohexane: All 6 carbon atoms in the ring are bonded to two other carbon atoms. Thus, it has 6 secondary carbon atoms.

2. 1,2,3,4-Tetramethylcyclohexane: The carbon atoms at positions 1, 2, 3, and 4 of the ring are tertiary (each bonded to two ring carbons and one methyl carbon). The carbon atoms at positions 5 and 6 are secondary (each bonded to two ring carbons). The four methyl carbons are primary. Thus, it has exactly two secondary carbon atoms.

3. 1-Ethyl-2,3,4-trimethylcyclohexane: The ring carbons at positions 5 and 6 are secondary. Additionally, the $-CH_2-$ group of the ethyl substituent is also a secondary carbon (bonded to the ring and a methyl group). Thus, it has three secondary carbon atoms.

4. 2,4-Dimethylheptane: The structure is $CH_3-CH(CH_3)-CH_2-CH(CH_3)-CH_2-CH_2-CH_3$. The carbons at positions 3, 5, and 6 of the parent chain are secondary. Thus, it has three secondary carbon atoms.

Therefore, 1,2,3,4-Tetramethylcyclohexane is the only compound with exactly two secondary carbon atoms.

Answer: 1,2,3,4-Tetramethylcyclohexane