In a multi-threaded server application, opening a separate database connection for each thread (option c) can lead to issues like resource contention and exhaustion. This approach can consume a significant number of resources and potentially cause performance problems. The other options (a, b, and d) do not accurately describe the issues associated with this practice.

In a financial application, precision is crucial. The double data type can store decimal values but may not provide the necessary precision due to its limited number of significant digits. BigDecimal is preferred in such scenarios because it offers arbitrary precision and is ideal for financial calculations where rounding errors need to be minimized. float and long do not provide the required precision for financial calculations.

In Java, the Future interface represents the result of an asynchronous computation. It allows you to retrieve the result or handle exceptions once the computation is complete. A Callable is used to perform a task and return a result, and a Runnable is used to represent a task that can be executed asynchronously, but neither of them directly represents the result of the computation. The Executor interface is used to execute tasks, not represent results.

The @FunctionalInterface annotation is used to indicate that an interface is intended to be used with lambda expressions. It serves as a marker for functional interfaces, making it clear that they are designed to be used in lambda expressions.

The sleep() method in Java is used to temporarily pause the execution of a thread for a specified amount of time. It's a way to introduce delays in a program and is often used for synchronization or timing purposes. The other options are not used for pausing threads in this manner.

The finally block in Java exception handling is used to ensure that a certain block of code is executed regardless of whether an exception is thrown or not. It is typically used to perform cleanup actions, such as closing resources.

In Java, the if keyword is used to test a condition. It allows you to execute a block of code only if the condition specified inside the parentheses evaluates to true. The if statement is fundamental for implementing conditional logic in Java programs.

In Java, the rows of a two-dimensional array are stored sequentially in memory. This means that the elements of each row are stored one after the other in memory, making it more efficient for accessing elements in a row. This sequential storage pattern is different from languages like C, where a 2D array is essentially an array of pointers to individual rows.

In Java, the syntax of a lambda expression uses the -> symbol. It separates the lambda parameters from the lambda body and is a distinctive feature of lambda expressions, making them easy to identify in code.

A Java class can implement an unlimited number of interfaces. This allows for multiple inheritance of method signatures, where the class must provide implementations for all the methods declared in the interfaces it implements.

Deadlock in multithreading occurs when two or more threads are waiting for resources acquired by each other. They cannot proceed as they are both holding resources the other needs, resulting in a deadlock situation.

In Java, the clear() method is used to remove all the mappings from a Map. It does not remove the map itself but makes it empty, removing all key-value pairs. The other options do not perform this specific function.