Understand the assertion

Assertion (A): Cis form of alkene is found to be more polar than the trans form.

This is generally true for disubstituted alkenes like $2$-butene.

• In the cis isomer, the bond dipoles are on the same side, so they add up.
• In the trans isomer, the bond dipoles are in opposite directions, so they tend to cancel.

Therefore, the cis isomer is more polar than the trans isomer.

Understand the reason

Reason (R): Dipole moment of trans isomer of 2-butene is zero.

Consider trans-2-butene:

$\mathrm{CH_3-CH=CH-CH_3}$

In the trans form, the two $\mathrm{CH_3}$ groups lie on opposite sides of the double bond. Because of symmetry, the individual bond dipole moments cancel out.

Hence,

$\mu(\text{trans-2-butene}) = 0$

So, the reason is also true.

Check whether the reason explains the assertion

Since the trans isomer has zero dipole moment due to cancellation, while the cis isomer has a non-zero dipole moment due to partial addition of dipoles, this directly explains why the cis form is more polar than the trans form.

Therefore, (R) is the correct explanation of (A).

Evaluate the options

• A: (A) true, (R) false $\rightarrow$ Incorrect
• B: (A) false, (R) true $\rightarrow$ Incorrect
• C: Both true and (R) correctly explains (A) $\rightarrow$ Correct
• D: Both true but (R) not correct explanation $\rightarrow$ Incorrect

Final answer

The correct option is:

$\boxed{\text{C}}$

Comparison with stored answer

Stored correct answer = C

My derived answer = C

So, they agree.