Air entrainment in the concrete increases a) workability b) strength c) the effects of temperature variations d) the unit weight

It used to increase the workability , but it will reduce the strength and unit weight

there are two main purposes of air entrained concrete.

1)to replace water by air so as to reduce the risks of freezing of water in cold regions thus increasing the durability of concrete

2) To increase the workability of concrete

i think the workability increases. due to the increase in air entrainment the temperature in the building decreases. strength and unit weight decreases

The purpose of adding air entrainment in concrete is to protect it from cracks due to freezing & thawing cycles.Compressive strength of concrete is inversely proportional to the workability of concrete. When workability of concrete increases, its compressive strength decreases.

So, to maintain both, the desired compressive strength and workability of concrete is considered while doing mix design so that the desired compressive strength is achieved with required amount of air entraining agent  together with workability.

In general, the compressive strength of concrete is reduced by air entrained admixture. The amount of reduction in strength depends on many factors like mix proportions, cement content, water cement ratio, type & grading of concrete as well as the amount of air entrainer.                                                          

Normal reduction in strength of concrete with the addition of air entrainment varies from3 to7 percent. This variation of strength shall be considered in the mix design of concrete. It is recommended that trial mix designs should be done to find the exact variation of strength with the use of air entrainment and based on findings, suitable corrections should be done in mix design to ensure desired strength.  

The Purpose of adding air entrainment  materials in concrete is to protect the concrete from cracks due to freezing.  Unfortunately there is no free ticket, when we add air to concrete we are reducing the amount of concrete by volume thus the unit weight will drop accordingly.

from other side adding air will also increase slump of concrete which mean higher workability, also including those air pebbles in the concrete will lower the compression of the concrete which must be considered in the design of the concrete.

Option C . it will increase the effects of Temperature variations ...

regarding concrete to be used in below4c degrees which will be subjected to freeze and thaw effect the perfect solution against the increase of water volume due to freeze is the air entraiment

  by using special admix to concrete mixture.

on the other hand of course it will be subjected to reduction in strength and more work-ability but if calculated the correct amount of air-entraiment admix the effect will be minimum and will not have big effect, conclusion the main purpose of  entering air inside concrete is to resist freeze and thaw effect mainly in road slabs and paving .

In most cases we refer to solid concrete but this action of creating small air bubbles in the concrete increases workability and durability reducing unnecessary  cracks  due to heat or cold as in summer and winter weathered world locations.

Workability is the only correct answer unless there are other certain types of admixtures or additives that might affect he workability in a reverse way...in that case, workability increment won;t be changed significantly.