When is di- used in the name of a hydrocarbon? when the main chain contains two carbon atoms when there are two different substituents when there are two main chains of equal length when there are two identical substituents

Correct answer is 
when there are two identical substituents
for example 1,2 - dichloroethane. 

Answer : The correct option is, when there are two identical substituents.

Explanation :

The basic rules for naming of organic compounds are :

First select the longest possible carbon chain.  When the main chain contains two carbon atoms then the name of the main chain is, eth-.The longest possible carbon chain should include the carbons of double or triple bonds. The naming of alkane is done by adding the suffix -ane, alkene by adding the suffix -ene, alkyne by adding the suffix -yne and carboxylic acid by adding the suffix -oic acid. The numbering is done in such a way that first carbon of double or triple bond gets the lowest number. The carbon atoms of the double or triple bond get the preference over the other substituents present in the parent chain. If two or more similar alkyl groups or identical substituents are present in a compound, the words di-, tri-, tetra- and so on are used to specify the number of times of the alkyl groups in the chain.

Hence, the correct option is, when there are two identical substituents.