An embargo is often used
a. to raise the cost of imported goods
b. to increase trade
c. to restrict the number of units of a particular imported good
d. for political reasons

An embargo is often used d. For political reasons

Explanation:

Trade of goods between countries increases the economy of the country.  An embargo is a "government order" under in which trade of goods between countries or specific goods gets restricted. It is usually a result of an unfavorable political or economic condition between two or more countries.  This decision is usually taken to force the governing people of the country to act on the issue because of which embargo decision was taken.  

Embargo can have negative effects on the country's economy. Like a trade embargo would restrict export and import from or to country/countries.

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