The market demand curve for a normal good will shift to the left (decrease) when the price of a substitute good ▼ increases decreases , the price of a complementary good ▼ decreases increases , consumer income ▼ increases decreases , and population ▼ decreases increases .

1. decreases 
2. increases3. decreases
4. decreases  
The answers here require you to understand the terms involved. So let's look at the options and see what is what.  
1. The price of a substitute good â–Ľ increases decreases​ 
* A substitute good is some good that can be used as a substitute for another good. So if that substitute becomes cheaper, it will be used more as a substitute for the original good. So the answer is "decreases"  
2. The price of a complementary good ▼ decreases increases​,
 * A complementary good is a good that's used in conjunction with another good. Something like milk and cookies. As more cookies are consumed, more milk is desired to go along with the cookies. So increasing the price of the complementary good will decrease the demand of the other good. So the answer is "increases"  
3. Consumer income â–Ľ increases decreases 
* If the consumer has less money to spend, then spending on non-essential goods will decrease. So the answer is "decreases".  
4. Population â–Ľ decreases increases 
* A smaller population is a reduced consumer base, so fewer goods are purchased. The answer is "decreases"