The maximum theoretical signal gain of chip-based TxBF and MRC depends on the number of antennas and the channel conditions. TxBF (transmit beamforming) is a technique that focuses the transmitted signal in the direction of the intended receiver, resulting in higher signal strength and less interference. MRC (maximum ratio combining) is a technique that combines the signals received by multiple antennas in an optimal way, resulting in higher signal-to-noise ratio and better performance. The theoretical gain of TxBF and MRC can be calculated as

GTxBF=10log10(N)

and

GMRC=10log10(N)

where N is the number of antennas. Therefore, the total gain of TxBF and MRC is

Gtotal=GTxBF+GMRC=20log10(N)

However, this is the ideal case and assumes perfect channel conditions and alignment. In reality, the gain is lower due to factors such as channel fading, antenna spacing, and feedback delay. According to the CWDP study guide1, the typical gain of TxBF and MRC is about 4-6.5 dBi for 2 antennas, 7-10 dBi for 3 antennas, and 10-13 dBi for 4 antennas. Therefore, the answer is D. Reference: 1: CWDP Certified Wireless Design Professional Official Study Guide: Exam PW0-250, Chapter 8, page 267.