**Explanation:**

The correct answer is option **b) Janata Party**.

The Janata Party provided two Prime Ministers in a span of two years. Let's delve into the details to understand why this is the correct answer.

**1. Formation of the Janata Party:**
The Janata Party was formed in 1977 as a political alliance of several opposition parties with the primary objective of defeating the ruling Congress party, which had been in power for several years. It was a coalition of various parties, including the Bharatiya Lok Dal, the Congress (O), the Socialist Party, and several regional parties.

**2. General Elections of 1977:**
In the general elections held in 1977, the Janata Party emerged as the winner, securing a significant mandate. This was a historic event as it marked the end of the Congress party's uninterrupted rule since India's independence in 1947. The Janata Party's victory was seen as a symbol of the people's desire for change and a break from the dominance of a single party.

**3. Prime Ministers from the Janata Party:**
Following their victory in the elections, the Janata Party appointed two Prime Ministers within a span of two years.

- **Morarji Desai (1977-1979):** After the elections, Morarji Desai, a senior leader of the Janata Party, was chosen as the Prime Minister of India. He took office on 24th March 1977. Morarji Desai was a veteran politician and had previously served as the Deputy Prime Minister and Finance Minister under Prime Minister Indira Gandhi. As Prime Minister, he implemented several reforms and policies, including the relaxation of press censorship and the abolition of the Privy Purse for former rulers of princely states.

- **Charan Singh (1979-1980):** In 1979, internal disagreements within the Janata Party led to the downfall of the Morarji Desai government. Charan Singh, another senior leader of the Janata Party, broke away and formed a new party called the Janata Party (Secular). With the support of the Congress party, Charan Singh became the Prime Minister on 28th July 1979. However, his government lasted only a short period, and he resigned in January 1980.

**4. Conclusion:**
In conclusion, the Janata Party provided two Prime Ministers in a span of two years. Morarji Desai served as the Prime Minister from 1977 to 1979, followed by Charan Singh's brief tenure from 1979 to 1980. The Janata Party's time in power was marked by significant political developments and changes in India's governance, making it an important period in the country's political history.