Which of the following statements is true?

a. Aromatic compounds are less reactive than alkenes.
b. Aromatic compounds are more reactive than alkenes.
c. Benzene tends to undergo addition rather than substitution reactions.
d. Benzene is more reactive than cyclohexane.

A is true.

I THINK D is true but check it out.

To determine which statement is true, let's analyze each statement one by one:

a. Aromatic compounds are less reactive than alkenes.
To determine the reactivity of aromatic compounds compared to alkenes, we need to look at the differences in their chemical structures and reactivity patterns. Alkenes are known for their characteristic double bond, which allows them to undergo addition reactions readily. Aromatic compounds, on the other hand, have a ring-like structure with delocalized pi-electrons, which imparts them with high stability and less reactivity compared to alkenes. Therefore, statement (a) is true.

b. Aromatic compounds are more reactive than alkenes.
As explained above, aromatic compounds are actually less reactive than alkenes due to their higher stability. Therefore, statement (b) is false.

c. Benzene tends to undergo addition rather than substitution reactions.
This statement is false. Benzene is well-known for its preference for substitution reactions rather than addition reactions. The aromatic ring in benzene is stabilized by the delocalization of the pi-electrons, which makes it less reactive towards addition reactions. Instead, benzene tends to undergo electrophilic aromatic substitution reactions where a substituent replaces one of the hydrogen atoms on the aromatic ring.

d. Benzene is more reactive than cyclohexane.
Comparing the reactivity between benzene and cyclohexane, we find that benzene is generally more reactive than cyclohexane. Cyclohexane is an alkane with C-C single bonds, making it relatively unreactive. However, benzene's aromatic ring with delocalized pi-electrons gives it higher reactivity. Therefore, statement (d) is true.

In summary, the correct statement is:

d. Benzene is more reactive than cyclohexane.