He is always standing up for the weak and oppressed.

Answer: Option B

The Correct Answer is Option B: Championing the cause of

Here's why:

* The idiom "standing up for" means to defend or support someone or something, especially when they are being criticized or attacked.

* Let's break down the options:
* A: Boosting the claims of - This means strengthening or promoting someone's arguments, which isn't quite the same as defending the weak.
* B: Championing the cause of - This means to publicly support and defend a cause or person. This fits perfectly with "standing up for".
* C: Seeking help of others for - This means asking for assistance, which is not implied in the original sentence.
* D: Moving about with - This means physically being with someone, which has nothing to do with support or defense.

* Therefore, "Championing the cause of" (Option B) is the best and most accurate synonym for "standing up for" in this context.