For transmitting a modulated signal of wave length \( \lambda \), the antenna should have minimum size :
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( \displaystyle\frac{\lambda}{4} \)
An antenna radiates a signal efficiently only when its size is comparable to the wavelength of the signal; a much shorter antenna sends out very little power. The minimum size accepted for efficient radiation is a quarter of the wavelength:
\( l_{\mathrm{min}} = \lambda/4 \)
This is also why a low-frequency message signal is first put on a high-frequency carrier (modulation): the shorter carrier wavelength makes an antenna of practical size possible.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Size of the Antenna or Aerial – “For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least λ/4 in dimension) so that the antenna properly senses the time variation of the signal.”