According to the ITU-T G.

GPON Wavelengths :

GPON networks use specific wavelengths for upstream and downstream transmission:

Downstream (OLT to ONU) : 1490 nm.

Upstream (ONU to OLT) : 1310 nm.

Optional CATV overlay : 1550 nm (used for video services).

Why 1490 nm?

The ITU-T G.984.x standards specify 1490 nm as the wavelength for downstream data transmission in GPON networks. This wavelength is chosen for optimal performance in fiber-optic communication.

Other Options :

850 nm : Used in multimode fiber for short-distance communication, not in GPON.

1310 nm : Used for upstream transmission in GPON.

1550 nm : Optional wavelength for CATV overlay, not for standard downstream data.

Thus, the correct answer is A .

References:

HCIA Huawei ACCESS Official Documentation , Chapter: GPON Wavelengths.

ITU-T G.984.x Standards for GPON .