The price at which an article is purchased is called its: -
(a) S.P.
(b) C.P.
(c) Gain
(d) Loss

Hint: Understand the definitions of each term provided in the options and find the correct answer. Consider the full form of S.P. as the selling price and the full form of C.P. as the cost price.

Complete step by step answer:
Here, we have been provided with a statement ‘the price at which an article is purchased is called its’, and we are asked to complete this statement with the provided options. But first, let us learn about the definitions of these terms provided in the options one – by – one. So, we have,
(a) S.P.: - The term S.P. stands for the selling price. It is defined as the price at which any good is sold by a person to the buyer. It is also known as a list price, quoted price, sale price or market price.
(b) C.P.: - The term C.P. stands for cost price. It is defined as the price at which any good is brought by a merchant or retailer.
(c) Gain: - The term gain is defined as the difference in selling price and cost price. If the S.P. of an article is greater than its C.P. then we say that gain occurs in that article. It is also called the profit and is calculated by using the formula given as: - Gain = S.P. – C.P.
(d) Loss: - The term loss is defined as the difference of cost price and selling price. If the C.P. of an article is greater than its S.P. then we say that loss occurs in that article. Loss is calculated by using the formula given as - Loss = C.P. – S.P.
On reading the definitions of all the terms provided in the options we can conclude that option (b) is the correct answer.

Note:
 One must remember the definitions of basic terms like S.P., C.P., Gain, Loss, Gain%, Loss%, etc. to solve the above question. In case options are not given, we need to use out memorized data to get the answer. Note that the term S.P. and C.P. are interrelated with each other. For example: - if a seller sells any article to a customer for Rs. x then this price is C.P. for the customer and S.P. for the seller.