for 1 will go with A as it follows if/then rule
for 2 will go with D for //ism sake

It is possible that future spacecraft will be slowly propelled through almost empty space within the solar system, not by jets of gas, but by the force of photons of sunlight pushing on thin plastic sails
a. It is possible that future spacecraft will be slowly propelled
b. It is possible for future spacecraft to be propelled slowly
c. Future spacecraft could be propelled slowly possibly
d. Future spacecraft could possibly slowly be propelled
e. It could be possible that future spacecraft be propelled slowly

It is passive voice sentence.... B

4) Income tax indexing is intended to eliminate the constant tax increase that results as inflation pushes individuals into higher tax brackets
a. constant tax increase that results as inflation pushes individuals
b. constant tax increase as a result of individuals being pushed by inflation
c. constant tax increase because inflation pushes individuals
d. constantly increasing taxes that result because of inflation pushing individuals
e. constantly increasing taxes because individuals are pushed by inflation

5) It is known that consuming too much sodium makes a contribution to hypertension, which is a factor in half the deaths in the United States each year
a. It is known that consuming too much sodium makes a contribution to hypertension
b. The consumption of too much sodium is known to have made a contribution to hypertension
c. Consuming too much sodium is known to contribute to hypertension
d. Contributing to hypertension is the consuming of too much sodium
e. Known to contribute to hypertension is the consumption of too much sodium

Answer for the 3rd question

3 - A

4-A and 5-C