The key point about "gradually" is the idea of grades, or degrees, or small steps. If someone says that "the temperature declined gradually from 20° to 10°" I have a mental image. I imagine that I'm looking at a thermometer and actually am able to notice that it is reading 20, then 19, then 18, then 17, ... then 10. But, yes, in order to do that, the temperature must decline slowly. So I would answer #2 "true." I believe that "gradually" implies steadily--the movement is always in the same direction, without reversals. So I would answer #1 "true" as well.

The second one is by definition always true. In this context, "slowly" and "gradually" are synonymous. The first one may or may not be true depending on the circumstances. "Steadily" has to do with the consistency of the decline rather than its speed. Something could gradually decline over a long period of time. I suspect that whoever wrote the questions intended for the first one to be false, but he or she doesn't seem to have firm understanding of the meaning of "steadily."

Decline = to become smaller, fewer, less. Steadily= in a regular and even manner. 2 in a controlled or unwavering manner. Gradually= In a gradual way by degrees. (by gradients) So whatever X is declining gradually or steadily (both synonymous.) Would have a direct bearing on which statement is the best to use. The easiest example to use is a hill. If it declines gradually it falls or deceases in its climb-ability , walkability by a regular smooth steady consistent amount over set measured distances. For its entire route/length/distance from top to bottom. If it declines steadily this will be similar and often used to mean the same. But a hill is not man made and evenly constructed or calculated for mans walkability. Instead it has good parts and bad parts for walking. In this instance declines gradually by measured gradients is not the best description. A man made road up and down a hill designed and constructed by engineers and road builders. Would therefore be declining gradually. To allow traffic cars horses, donkeys to be able to gradually climb up and down. Conclusion the first sentence is correct True. For a man made engineered road up and down a hill/decline (both synonymous. In this context) The second sentences is incorrect False, For a man made road up a hill. Slowly is also connected at times by speed. (but it is used informally and colloquially to mean the same. Sentence 1 for academia sentence 2 for informality and daily speaking) Also whatever is being measured will determine which sentence is best. If measuring the time/speed of something decaying. Sometimes you will be interested in its speed of decline but would probably use another word like decay.. Back to the hill example engineers will never make a hill that declines (engineer a decline) that falls too slowly or too fast. That would be dangerous and/or unusable unworkable. PLUS: you have not shown us the context.

2nd sentence is correct. if something falls gradually means it is falling / decreasing at a slow pace. If something decreases steadily means something is falling/decreasing at a fixed rate. It can either be slow or fast.

Steadily