Why was great britain's slave trade act of 1807 ineffective at fully abolishing slavery in the british empire?

It was ineffective because it declared that people who participated in trading slaves would be punished, not those who already owned slaves. It didn't outlaw slavery, it just outlawed trading slaves, so people still had slaves and often broke the law to procure them, after which they would be safe if they weren't caught during the trade.

Which reforms did gandhi initially promote when he joined the national congress

Your answer is d)magnesium