Answer:
Relation between initial speed of bullet and height h is given as

Explanation:
As we know that system of block and bullet swings up to height h after collision
So we have

so we have

so speed of the block + bullet just after the impact is given by above equation
Now we also know that there is no force on the system of bullet + block in the direction of motion
So we can use momentum conservation

now we have

The best answer would be the 4th choice. "They help scientists explain concepts that are difficult to observe, this also covers the first answer which helps the scientist to answer complex questions. A scientific model is not used prove scientific laws as they may not always have all the data to prove so, instead it is used to allow them to explain better concepts revolving around science through research and may also allow them to predict results based on the accumulation of data and analyzing the trend of this resulting information.