Final answer:
The internal alkyne among the given options is (c) acetylene.
Step-by-step explanation:
The internal alkyne among the given options is (c) acetylene.
An internal alkyne is a hydrocarbon with a triple bond between two carbon atoms that are not at the ends of the carbon chain. Acetylene, also known as ethyne, is the simplest internal alkyne as it consists of two carbon atoms connected by a triple bond.
Other options, such as 1-hexyne (a), 3,3-dimethyl-1-hexyne (b), and 4,5-dimethyl-2-hexyne (d), contain a triple bond between a carbon atom at the end of the carbon chain and another carbon atom within the chain, making them terminal alkynes.